Many games with those invasive rootkit DRMs simply don't work.

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 :)

Anticheat-protected games just don’t work on Steam Deck (usually just refusing to connect).

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.