As you already know  Exchange Server 2016 will be entering Extended Supporton October 14th 2020..
During that period it will receive only Critical and will be no further release of CUs, with the last planned CU to be  released in December 2020.
One of the many concerns regarding that transition is that at the moment there is no free Exchange Server 2019 license for just managing the exchange attributes on hybrid deployments as there is for Exchange 2016.
According to the Product Group that specific license for the Exchange Server 2019 will be release after 14th October.
Also as a heads-up the product group is working on a solution that will remove the need of those on-premise “management attributes| only servers.
Microsoft encourages Exchange Server 2016 customers to adopt CU19 as soon as possible to ensure uninterrupted delivery of any future security related fixes. After December 15, 2020, only CU19 or its successors will receive critical updates. During the Extended Support phase, only the latest CU is eligible to receive critical updates once the standard 3 month transition period of the prior CU has lapsed. 
Critical updates will continue to be made available via Windows Update and the Microsoft Download Center. Additional lifecycle information for all Microsoft products is available on docs.microsoft.com.
These are great news since is the first time that a working plan for the retirement of those on-premise server serving only for management is announced.

This cumulative update is a security update. It includes fixes for nonsecurity issues and all previously released fixes for security and nonsecurity issues

Cumulative Update 6 for Microsoft Exchange Server 2019 was released on June 16, 2020. This cumulative update is a security update. It includes fixes for nonsecurity issues and all previously released fixes for security and nonsecurity issues. These fixes will also be included in later cumulative updates for Exchange Server 2019. 
This update also includes new daylight saving time (DST) updates for Exchange Server 2019. For more information about DST, see Daylight Saving Time Help and Support Center.
Known issues in this cumulative update
In multidomain Active Directory forests in which Exchange is installed or has been prepared previously by using the /PrepareDomain option in Setup, this action must be completed after the /PrepareAD command for this cumulative update has been completed and the changes are replicated to all domains. Setup will try to run the /PrepareAD command during the first server installation. Installation will finish only if the user who initiated Setup has the appropriate permissions.

Notes If you are upgrading from Cumulative Update 2 for Exchange Server 2019 or a later cumulative update for Exchange Server 2019 to Cumulative Update 6 for Exchange Server 2019, there’s no need to run the /PrepareAD or /PrepareDomain. No additional actions (prepareAD, prepareDomain, or assigning permissions) are required. If you have ever skipped a Cumulative Update (for example, you are upgrading from an earlier version before Cumulative Update 2 for Exchange Server 2019), or this is a first Exchange Server installation in the AD, then this Known Issue section should be taken care of. About the /PrepareDomain operation in multidomain:

The /PrepareDomain operation automatically runs in the Active Directory domain in which the /PrepareAD commandis run. However, it may be unable to update other domains in the forest. Therefore, a domain administrator should run the /PrepareDomain in other domains in the forest. About the permission question:

As the /PrepareAD is triggered in Setup, if the user who initiates Setup isn’t a member of Schema Admins and Enterprise Admins, the readiness check will fail and you receive the following error messages.



To avoid the errors, either the user should join Schema Admins and Enterprise Admins groups or another user in Schema Admins and Enterprise Admins groups manually runs the /PrepareAD for this Cumulative Update first. Then, the Exchange admin user can start Setup. Autodiscover Event ID 1 occurs after you install Cumulative Update 3 for Exchange Server 2019. For more information, see KB 4532190. Issues that this cumulative update fixes
This cumulative update also fixes the issues that are described in the following Microsoft Knowledge Base articles:
4559441 Foreign language characters set in RejectMessageReasonText of a transport rule aren’t shown correctly in Exchange Server 2019 4547707 Enable piping for Restore-RecoverableItems in Exchange Server 2019 4549689 HMA EvoSTS certificate rollover causes authentication prompts due to stalled key on worker process spawn (warmup phase) in Exchange Server 2019 4559446 Changes to Outlook on the web blocked file extensions and MIME types in Exchange Server 2019 4559440 Export to a PST for an eDiscovery search fails Exchange Server 2019 4559439 EAS creates failure report if a message with unknown recipients is in Drafts in Exchange Server 2019 4559442 2080 Events caused by empty values in HKLM\SYSTEM\CurrentControlSet\Services\MSExchange ADAccess\Instance0 in Exchange Server 2019 4559438 Edge Transport server hangs in Exchange Server 2019 4559443 Managed Folder Assistant fails with Event ID 9004 NotInBagPropertyErrorException in Exchange Server 2019 4559437 PR_RECIPIENT_ENTRYID is computed if no email address or type in Exchange Server 2019 4559444 Conversion from HTML to RTF removes non-breaking space in Exchange Server 2019 4559436 Attachments with properties (like Azure Information Protection labels) not always matching in Exchange Server 2019 4559435 Introduce an OrganizationConfig flag to enable or disable recipient read session in Exchange Server 2019

This cumulative update is a security update. It includes fixes for nonsecurity issues and all previously released fixes for security and nonsecurity issues




Cumulative Update 6 for Microsoft Exchange Server 2019 was released on June 16, 2020. This cumulative update is a security update. It includes fixes for nonsecurity issues and all previously released fixes for security and nonsecurity issues. These fixes will also be included in later cumulative updates for Exchange Server 2019.


This update also includes new daylight saving time (DST) updates for Exchange Server 2019. For more information about DST, see Daylight Saving Time Help and Support Center.


Known issues in this cumulative update





In multidomain Active Directory forests in which Exchange is installed or has been prepared previously by using the /PrepareDomain option in Setup, this action must be completed after the /PrepareAD command for this cumulative update has been completed and the changes are replicated to all domains. Setup will try to run the /PrepareAD command during the first server installation. Installation will finish only if the user who initiated Setup has the appropriate permissions. Notes
If you are upgrading from Cumulative Update 2 for Exchange Server 2019 or a later cumulative update for Exchange Server 2019 to Cumulative Update 6 for Exchange Server 2019, there’s no need to run the /PrepareAD or /PrepareDomain. No additional actions (prepareAD, prepareDomain, or assigning permissions) are required. If you have ever skipped a Cumulative Update (for example, you are upgrading from an earlier version before Cumulative Update 2 for Exchange Server 2019), or this is a first Exchange Server installation in the AD, then this Known Issue section should be taken care of. About the /PrepareDomain operation in multidomain: The /PrepareDomain operation automatically runs in the Active Directory domain in which the /PrepareAD commandis run. However, it may be unable to update other domains in the forest. Therefore, a domain administrator should run the /PrepareDomain in other domains in the forest.
About the permission question: As the /PrepareAD is triggered in Setup, if the user who initiates Setup isn’t a member of Schema Admins and Enterprise Admins, the readiness check will fail and you receive the following error messages.

To avoid the errors, either the user should join Schema Admins and Enterprise Admins groups or another user in Schema Admins and Enterprise Admins groups manually runs the /PrepareAD for this Cumulative Update first. Then, the Exchange admin user can start Setup.
Autodiscover Event ID 1 occurs after you install Cumulative Update 3 for Exchange Server 2019. For more information, see KB 4532190.

Issues that this cumulative update fixes





This cumulative update also fixes the issues that are described in the following Microsoft Knowledge Base articles:


4559441 Foreign language characters set in RejectMessageReasonText of a transport rule aren’t shown correctly in Exchange Server 2019 4547707 Enable piping for Restore-RecoverableItems in Exchange Server 2019 4549689 HMA EvoSTS certificate rollover causes authentication prompts due to stalled key on worker process spawn (warmup phase) in Exchange Server 2019 4559446 Changes to Outlook on the web blocked file extensions and MIME types in Exchange Server 2019 4559440 Export to a PST for an eDiscovery search fails Exchange Server 2019 4559439 EAS creates failure report if a message with unknown recipients is in Drafts in Exchange Server 2019 4559442 2080 Events caused by empty values in HKLM\SYSTEM\CurrentControlSet\Services\MSExchange ADAccess\Instance0 in Exchange Server 2019 4559438 Edge Transport server hangs in Exchange Server 2019 4559443 Managed Folder Assistant fails with Event ID 9004 NotInBagPropertyErrorException in Exchange Server 2019 4559437 PR_RECIPIENT_ENTRYID is computed if no email address or type in Exchange Server 2019 4559444 Conversion from HTML to RTF removes non-breaking space in Exchange Server 2019 4559436 Attachments with properties (like Azure Information Protection labels) not always matching in Exchange Server 2019 4559435 Introduce an OrganizationConfig flag to enable or disable recipient read session in Exchange Server 2019

This cumulative update includes fixes for nonsecurity issues and all previously released fixes for security and nonsecurity issues.

Known issues in this cumulative update
In multidomain Active Directory forests in which Exchange is installed or has been prepared previously by using the /PrepareDomain option in Setup, this action must be completed after the /PrepareAD command for this cumulative update has been completed and the changes are replicated to all domains. Setup will try to run the /PrepareAD command during the first server installation. Installation will finish only if the user who initiated Setup has the appropriate permissions.
Notes If you are upgrading from Cumulative Update 13 for Exchange Server 2016 or a later cumulative update for Exchange Server 2016 to Cumulative Update 17 for Exchange Server 2016, then there’s no need to run the /PrepareAD or /PrepareDomain. No additional actions (prepareAD, prepareDomain, or assigning permissions) are required. If you have ever skipped a Cumulative Update (for example, you are upgrading from an earlier version before Cumulative Update 13 for Exchange Server 2016), or this is a first Exchange Server installation in the AD, then this Known Issue section should be taken care of. About the /PrepareDomain operation in multidomain:
The /PrepareDomain operation automatically runs in the Active Directory domain in which the /PrepareAD command is run. However, it may be unable to update other domains in the forest. Therefore, a domain administrator should run the /PrepareDomain in other domains in the forest. About the permission question:
As the /PrepareAD is triggered in Setup, if the user who initiates Setup isn’t a member of Schema Admins and Enterprise Admins, the readiness check will fail and you receive the following error messages.To avoid the errors, either the user should join Schema Admins and Enterprise Admins groups or another user in Schema Admins and Enterprise Admins groups manually runs the /PrepareAD for this Cumulative Update first. Then the Exchange admin user can start Setup.
Autodiscover Event ID 1 occurs after you install Cumulative Update 14 for Exchange Server 2016. For more information, see KB 4532190. Issues that this cumulative update fixes
This cumulative update fixes the issues that are described in the following Microsoft Knowledge Base articles:
4559444 Conversion from HTML to RTF removes non-breaking space in Exchange Server 2016 4559435 Introduce an OrganizationConfig flag to enable or disable recipient read session in Exchange Server 2016 4547707 Enable piping for Restore-RecoverableItems in Exchange Server 2019 and 2016 4559436 Attachments with properties (like Azure Information Protection labels) don’t always match in Exchange Server 2016 4559437 PR_RECIPIENT_ENTRYID is computed if no email address or type in Exchange Server 2016 4559438 Edge Transport server hangs in Exchange Server 2016 4559439 EAS creates failure report if a message with unknown recipients is in Drafts in Exchange Server 2016 4559440 Export to a PST for an eDiscovery search fails in Exchange Server 2016 4559441 Foreign language characters set in RejectMessageReasonText of a transport rule aren’t shown correctly in Exchange Server 2016 4559442 2080 Events caused by empty values in HKLM\SYSTEM\CurrentControlSet\Services\MSExchange ADAccess\Instance0 in Exchange Server 2016 4549689 HMA EvoSTS certificate rollover causes authentication prompts due to stalled key on worker process spawn (warmup phase) in Exchange Server 2016 4559443 Managed Folder Assistant fails with Event ID 9004 NotInBagPropertyErrorException in Exchange Server 2016 4559446 Changes to Outlook on the web blocked file extensions and MIME types in Exchange Server 2016

This cumulative update includes fixes for nonsecurity issues and all previously released fixes for security and nonsecurity issues.

Known issues in this cumulative update
In multidomain Active Directory forests in which Exchange is installed or has been prepared previously by using the /PrepareDomain option in Setup, this action must be completed after the /PrepareAD command for this cumulative update has been completed and the changes are replicated to all domains. Setup will try to run the /PrepareAD command during the first server installation. Installation will finish only if the user who initiated Setup has the appropriate permissions.
Notes If you are upgrading from Cumulative Update 13 for Exchange Server 2016 or a later cumulative update for Exchange Server 2016 to Cumulative Update 17 for Exchange Server 2016, then there’s no need to run the /PrepareAD or /PrepareDomain. No additional actions (prepareAD, prepareDomain, or assigning permissions) are required. If you have ever skipped a Cumulative Update (for example, you are upgrading from an earlier version before Cumulative Update 13 for Exchange Server 2016), or this is a first Exchange Server installation in the AD, then this Known Issue section should be taken care of. About the /PrepareDomain operation in multidomain:
The /PrepareDomain operation automatically runs in the Active Directory domain in which the /PrepareAD command is run. However, it may be unable to update other domains in the forest. Therefore, a domain administrator should run the /PrepareDomain in other domains in the forest. About the permission question:
As the /PrepareAD is triggered in Setup, if the user who initiates Setup isn’t a member of Schema Admins and Enterprise Admins, the readiness check will fail and you receive the following error messages.To avoid the errors, either the user should join Schema Admins and Enterprise Admins groups or another user in Schema Admins and Enterprise Admins groups manually runs the /PrepareAD for this Cumulative Update first. Then the Exchange admin user can start Setup.
Autodiscover Event ID 1 occurs after you install Cumulative Update 14 for Exchange Server 2016. For more information, see KB 4532190. Issues that this cumulative update fixes
This cumulative update fixes the issues that are described in the following Microsoft Knowledge Base articles:
4559444 Conversion from HTML to RTF removes non-breaking space in Exchange Server 2016 4559435 Introduce an OrganizationConfig flag to enable or disable recipient read session in Exchange Server 2016 4547707 Enable piping for Restore-RecoverableItems in Exchange Server 2019 and 2016 4559436 Attachments with properties (like Azure Information Protection labels) don’t always match in Exchange Server 2016 4559437 PR_RECIPIENT_ENTRYID is computed if no email address or type in Exchange Server 2016 4559438 Edge Transport server hangs in Exchange Server 2016 4559439 EAS creates failure report if a message with unknown recipients is in Drafts in Exchange Server 2016 4559440 Export to a PST for an eDiscovery search fails in Exchange Server 2016 4559441 Foreign language characters set in RejectMessageReasonText of a transport rule aren’t shown correctly in Exchange Server 2016 4559442 2080 Events caused by empty values in HKLM\SYSTEM\CurrentControlSet\Services\MSExchange ADAccess\Instance0 in Exchange Server 2016 4549689 HMA EvoSTS certificate rollover causes authentication prompts due to stalled key on worker process spawn (warmup phase) in Exchange Server 2016 4559443 Managed Folder Assistant fails with Event ID 9004 NotInBagPropertyErrorException in Exchange Server 2016 4559446 Changes to Outlook on the web blocked file extensions and MIME types in Exchange Server 2016