stevewdj will be authoritativeon the answer.  Which probably depends on whether tehy are looking for a specific Schema or function in 2019 missing in 2016.  But I suspect they are referring to AD Schema, Forest and Domain levels.  Not specifically the Windows version, rather the functionality of AD.  Can Window 2016 AD run in a tree that is at the 2019 forest level?

It's just for authentication, so we are in fact only referring to the AD level. As far as I would expect there shouldn't be that much changes, in the end it's still just an LDAP-query. But that's exactly why I wonder it's written AD that comes with Server v2019. 

Hello,
   The version of Active Directory listed in the 3.7.3 documentation is the version that was tested and certified against. It is possible that an earlier version of AD could work correctly, but it was not tested and therefore would not be support or certified.

  Please be aware that Identity Governance 3.6.1 is no longer "supported" and 3.6.2 (3.6.2.5) is only "supported" until 31-March-2023. I would strongly recommend upgrading to 3.7.3 as quick as possible. For version 3.7.3, we only support upgrading from version 3.7.0 or 3.6.2 (3.6.2.5). If you are still on version 3.6.1 (per one of your other posts where you outlined that version 3.6.1 is being utilized), moving to 3.7.3 would require two (2) updates: 3.6.1 -> 3.6.2 -> 3.7.3 or 3.6.1 -> 3.7.0 -> 3.7.3

Sincerely,
Steven Williams
Principal Enterprise Architect
Micro Focus

Thanks, in fact we're still on 3.6.1... we will update asap.