FWIW it's fairly straightforward to set up FSR4 with Proton-GE nowadays, assuming you're comfortable with editing one config file or manually specifying an env var for the game[1]. I'm not sure if using an alternate version of Proton would be considered "native" though, or if you mean for the default version of proton (or for Linux builds of games specifically), but setting it up is a fairly straightforward process even for people who might prefer not to use the terminal if you use something like ProtonUp to manage the installation for you. I imagine that the process for using a custom Proton isn't much different on Bazzite and CachyOS, although I'm not sure whether it would be something commonly done on CachyOS given that they have their own Proton distribution.

[1]: I don't think there's a way to link to it directly, but `PROTON_FSR4_UPGRADE=1` (or a specific different version if you'd like) is documented in the README in this table: https://github.com/GloriousEggroll/proton-ge-custom#modifica...