While the support itself doesn’t necessarily mean OS-level toggle like the Steam Deck has right (which I believe is still at FSR 1), it still could be a nice touch for the titles that choose to implement FSR 4.
Maybe the community adds the OS-level support unofficially, somewhat similarly to Lossless Scaling right now (although perhaps for free). This is just my uneducated guesswork, please see this comment for proper expectations:
Optiscaler (with Windows games) also works if you need further tweaks or run into compatibility issues with the Proton level support.
That’s partially true, unfortunately some games do not work with the FSR4 upgrade via OptiScaler, and on top of that there’s a plausible risk of getting banned in games with anticheat