Also, Classic FreeCIV on the web looks like fun too, definitely have to find a weekend for a round. The Anti-cheat stuff is unfortunately a necessity for multiplayer games, as even Pokemon GO PvP cheats lamed up the game. lol
Cheers =)
It's software that tries to validate the environment the game is running in and is suddenly confronted with "wait, thats not a Windows system at all". And as you already argued, just ditching Anti-cheat is not an option.
So they either ignore Linux/Proton altogether or have to start actually implementing anti-cheat for a Linux/Proton environment. Even if they go with the second option (as e.g. EAC is, iirc), it simply takes time to implement.
Most of the popular anti cheats do work on Proton. EAC, Punkbuster, Battleye, and VAC all work fine, which I think covers most games with anti cheat. Unfortunately some of those require opt-in by the developer to work on Proton, and therefore some developers have neglected to opt in.
A handful of other games use their own anti cheat that does not work, and unfortunately those games have the resources to do that because they are very popular (CoD, Valorant).
Valve took a look at how hard Steam Machines bombed, and tried again with Proton. They saw some companies anti-cheat rootkits weren't working, and didn't let that stop them from making all the other games work.
Valve saw the light over a decade ago. Hell, I'd argue they always saw Linux as second-class, but worth not completely ignoring or actively breaking.