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
Yeh see I was thinking of FACEIT because I heard reports that Battlebit (which uses FACEIT) wouldnt work on the steam deck, but checking protondb it seems it runs fine, so I guess there are workarounds.
Wouldn't doubt that Ricochet and Vanguard have no support for it at all though
You're right, they talked about adding Faceit AC, but they still need time. Once they actually implement Faceit and and thereby block Steam Deck/proton users. At that point Battlebit simply won't work anymore. (Hopefully Steam'll refund my game or BB will have good community servers.)
Oh, my assumption was that they either found a way to make FACEIT work with proton or dropped it due to the lack of support. Seems odd that they'd plan to update a game that actively eliminates a section of the playerbase, especially for a multiplayer game from a relatively small studio that relies on having a big enough playerbase for the game to function
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.
I think there are a few other anti-cheats which can be problematic, but again I think it can be worked around