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Why not? Could you elaborate? I'd love to know more. I always had the feeling that supporting SteamOS basically meant that generic Linux Desktop support was almost implied because in the end it's almost always on Proton rather than native.

That is not true. Proton and steam linux runtime which are the components actually responsible to run games are literally the exact same code provided by the steam client.

https://github.com/ValveSoftware/steam-runtime

https://github.com/valvesoftware/proton

If you are a gamedev considering native support for linux then you are using the steam runtime (i.e. a debian container) anyways.

Or you just write a Windows program but explicitly target Proton. (Granted, either of those remain portable to at least any Linux with Steam installed.)