[chrisnight]

Yes, I have plenty of games from, e.g. the Epic Game Store on my steam deck, even in the steam home page, seamlessly.

Gamescope is even fully open-source, so you could remove the steam deck UI, and still run any game with the same performance benefits of not running it inside KDE. Of course also, you could flash a new OS on the device itself if you wanted to entirely remove Valve’s presence.