Valve has been busy getting more anti-cheat/DRM services working under Proton (Steam Play) ahead of the first Steam Deck devices shipping to consumers later this quarter. Their latest achievement is getting CEG DRM'ed games working, at least if you are willing to use Proton Experimental and Steam beta for the time being.
This should get games like Mafia II, Hitman: Absolution, Sniper Elite V2, GRID 2, FEAR 3, Call of Duty: Modern Warfare 2, F1 Race Stars, and numerous other games now working.
More details on this improvement for games using CEG DRM via this Proton issue tracker ticket .