Yeah, but that compatibility layer, and steam itself, is available for all (PC-based) Linux distributions, as far as I know. I don't doubt you'll run into problems if you want to use it on devices never meant to run games at all.
Even non-Steam games, installed through non-Steam platforms (like Heroic, which supports GOG), still have the option of using Valve's version of Proton, use the Steam runtime itself, or even apparently importing the game into Steam (I haven't tried that yet). Valve is not monopolizing Steam gaming, but sharing this support.