The problem with SDL's Wayland backend is that, as I understand it, you won't get any window decorations on common Wayland compositors like the Gnome one because Gnome expects every app to implement the window decorations itself (and the expected way to do this is by dragging in the entirety of Gtk, even if you're not using any of the other functionality of the toolkit). It looks like SDL are working on some kind of solution but it doesn't seem to be ready yet.

As it stands, though, it's probably not feasible to actually ship a game with Wayland support using SDL due to the potential support issues from people not running in fullscreen mode being stuck with a window that can't be moved, minimized or closed in the usual way.