Valve tries to make it easy to run a proton-compatible anti-cheat system which I believe is built in to steam libraries, but developers still have to choose to use it notably Microsoft seemed totally uninterested in using a proton compatible EAC for Halo Master Chief Collection last time I checked earlier this year. I read online it should have been easy to switch to the proton compatible EAC but some devs might not want to.

Halo: MCC specifically did get support for it a few months ago. But anti-cheat is still very dependent on the game.

That is nice to know thanks! I figured it would come eventually, I’m glad they added it.

Anti-cheat is usually a kernel extension, I don't think you can do that in Proton.

Edit: yes, I know EAC is Linux supported, but you can't use proton to run a Windows kernel extension.

Proton has some support for easy anti cheat. https://www.gamingonlinux.com/2022/02/proton-7-easy-anti-che...

It seems to be up to the developer, as many anti-cheat support Linux if the developer wants it to.

Elden Ring uses Easy Anticheat which works fine in Proton, but Black Desert also uses EAC and it doesn't. Phantasy Star Online 2 works with Proton-GE and it used GameGuard and now Uncheater.

Here's a crowdsourced list of games with anticheat that do or do not work with Proton: https://www.protondb.com/explore?selectedFilters=antiCheat