Hello @Ray ,
Thank you for posting here.
Windows server 2016 and Windows server 2019 are two adjacent operating systems, and the functional difference is not too great.
I think there should be no problem with pairing a Sever 2016 and a Server 2019 for DFS.
I am sorry, I did not find such an article to clarify this particular question.
From the following two links, we can see the new feature and important issues about Windows server 2016 and Windows server 2019 (it does not mention anything about DFS replication).
What's New in Windows Server 2016
https://learn.microsoft.com/en-us/windows-server/get-started/whats-new-in-windows-server-2016
What's new in Windows Server 2019
https://learn.microsoft.com/en-us/windows-server/get-started-19/whats-new-19
Release Notes - Important issues in Windows Server 2019
https://learn.microsoft.com/en-us/windows-server/get-started-19/rel-notes-19
Release Notes: Important Issues in Windows Server 2016
https://learn.microsoft.com/en-us/windows-server/get-started/windows-server-2016-ga-release-notes
Hope the information is helpful.
Best Regards,
Daisy Zhou