plenty of proton games support online multiplayer (natively or otherwise), its just games which use EAC thats the problem. Even then it can be fixed with (what I've heard) minimal work from the developer, but sadly thats still not quite universal.

I think there are a few other anti-cheats which can be problematic, but again I think it can be worked around

Depends on the game, and the anti-cheat software it uses.

EAC for Linux works

Another is peripheral support. I have a trackir which doesn't really work right in Linux on the games I'd like to use it. It also takes a lot more fiddling to get even supported devices (like my Logitech gaming wheel) working correctly.

And that's without even considering vr goggles...