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.