With recent releases, ITMS 8.5 and 8.6, we still support earlier 2003 and 2008 domains during our Microsoft Active Directory Imports.
As well, with recent domain versions like 2012 and 2016, all of these are working well. We recently did most of our tests on 2016 domains.
For us, it's just an LDAP protocol that domain structure should support, so AD Import works fine. Unless Microsoft introduces breaking changes in LDAP protocol in AD, we are not concerned about the versions of AD.
We haven't made any fixes in our code in years because MS shipped a new version of AD import.