This gets repeated a lot but there is at least one sort of meta XEP that specifies a common set to support. If you stick to the ~5* or so biggest clients they all support the important stuff.

* Conversations, Gajim, Dino, Kaidan, things on omemo.top with a passing grade.

Things which are still not default working everywhere would be voice/video call, but if you adjust your expectations to "IRC with file uploads, no need for a bouncer, and encryption" then it's fairly universal to support all that.