Ironically, the Wayland compositor used by the steam deck, gamescope, doesn't support native Wayland applications. Even weirder, it runs inside an X11 session.

To be fair, that includes a huge portion of the games available on a Steam Deck right now, given that Wine (and by extension Proton) currently only support running via XWayland. On the other hand, that's argument in favor of Wayland from my perspective; if something I want to run doesn't support Wayland, it doesn't actually stop me from using it.