With the release of Exchange 2019 CU14 today, a separate announcement was added to the release post to highlight that only CU23 is supported for Exchange 2016. Exchange Server 2016 is supported until the end of its support in 2025, however in order to receive support and to obtain Security Updates (SUs) you must be running CU23. CU22 and all older releases of Exchange 2016 are no longer supported. You may note that the typical Exchange support policy is N and N-1 for updates. This does not apply to the final version as the older CUs will no longer be able to be serviced and thus only the most recent CU is supported. The same process was also applied to Exchange 2013.
The Exchange Server supportability matrix also states the same. Under the Supported Versions and Builds section you will find:
Installing CU23 alone is not sufficient. You must also have the latest SU installed on all servers, and deploy Extended Protection.
Cheers,
Rhoderick
