As I plan upcoming AD upgrade (from 2008R2 to 2019) I face another problem: we're actually have AD Forest that consists of several sites, each of them on 2008R2 AD version. As we upgrade one site AD version from 2008R2 to 2019, other sites will remain on 2008R2 for quite site time until we'll see 2019 scheme works well and no problem appears. And now I doubt if the whole forest be functional and no problem arise when one site be on 2019 and other sites on 2008R2 AD scheme versions.
So the question is: in practice (that is, in real life) will the forest still be functional and usable as one site (and, eventually, one by one all other sites, too) be upgraded, while remaining sites still be on lower AD version? In general, the forest needed to have cross-sites auth, so no sophisticated features are needed, but anyway I'd better ask before jumping into the water :)