Even for AC, most AC is supported. Battle Eye and Easy Anti-cheat both work fine (with the proton patches that should be automatically installed). Maybe there's some custom AC that doesn't work, but I haven't found it yet. I'd guess Riot's doesn't if you want to play Valorant or LoL and want to install their root kit. I've had issues with The Finals (who just took a long time to update EAC but works fine now) and Squad (which is using a depricated C function that isn't included in glibc anymore, but is included with the Flatpak version of Steam so it's still playable with that) but they're solvable. I believe that's all.