Nope. Runs Linux and there are launchers for pretty much all the other games stores. It's just a significantly smoother process for things you have in Steam because of the work Valve has done on Linux compatibility.

You can pirate on the deck. It's like two extra steps compared to PC, but you can.

If you're not a Linux weirdo there are subreddits and people have made it easy, though you do have to use desktop mode to download games afaik.

If you're already a big linux weirdo, just ssh in and look at htop while playing any steam game in deck mode and figure out how the magic works yourself, trust me it's a lot of fun and the thing is really nice to explore because it's not locked down at all.