Valve is also pretty amenable to adding third-party games to the box as well. It's trivial to add games from GOG using Heroic and play them alongside regular Steam games.

My main "game console" is a NixOS box running Jovian, and I have made oodles of Flakes that I use to make custom non-Steam games that launch deterministically, and as far as I'm aware, Valve is perfectly fine with this (not that I particularly care if they are).