Came here to say SteamOS as well. I am surprised more people aren't saying it. But the thing is, the Steam Deck works so well I have to wonder how many people don't know/care the OS it runs, or maybe they have forgotten it's running Linux, or maybe they know but don't consider themselves "Linux Gamers" just because they are using it.
Sort of like how people playing on a Switch or PS3/4/Vita are technically FreeBSD gamers deep down.
I agree, it's great on the Steam Deck. I played for a bit on my laptop and it was fine there but it was a much better experience on the Deck.