Some publishers actually did remove the DRM just for the Deck version proving that DRM maybe isn't all that necessary when there's a will and manufacturer that doesn't bow down to ridiculous requirements :)
A few anticheat engines support Steam Deck specifically after working with Valve, but even then each individual game developer has to opt into it. (For example: Destiny 2 uses EasyAntiCheat which is Deck-compatible but they still refuse to whitelist it on Deck/Steam OS, and warn you may get banned if you try to sidestep the block)
Modern DRMs like Denuvo actually do tend to work in Proton/Wine (aka Steam OS’s compatibility layer) because they’re a wrapper around the game instead of a driver/debugger+game setup.
It's possible to give the user full control and the ability to give apps more permissions, while still sandboxing apps by default. Consider, for example, flatpak, which can make the host filesystem totally invisible to apps but also allows the user to pass in files via the file picker or to totally expose paths by overriding its filesystem context.
To be fair, even Android apps are reasonably locked down while still making it possible to install a proscribed emulator or Firefox or whatever
Which is why I stopped playing AAA games. I'm fine with avoiding them and having a machine I can actually control.
If phones were more open, we'd be free to mod our own games and back up our own save files, but we'd have to live with every random crap app we download being able to take over the whole machine permanently any time.