>There are countless ways to do kb+mouse sharing.

You realize that's worse, right? And to be clearer: core Wayland protocol does not have countless ways. It has zero.

Instead of a single protocol with the strong X11 reference X server the wayland compositors pick and choose between libinput, or libei, or libportal with the InputCapture PR, xdg-desktop-portal with the InputCapture interface, some I've probably missed, or maybe you have nothing at all (weston). It's a gamble if your choice of desktop environment and it's wayland compositor's non-core wayland protocols will match up with those the developer for $software chose. On X11 linux everything that works somewhere works everywhere. With the various waylands if you stay within your desktop's ecosystem you'll probably not notice, but go beyond it and you will.

Each wayland desktop pretty much runs it's own compositor with it's own set of third party libs because the wayland core protocol spec is very minimal. I would say incomplete. ref: https://wayland.app/protocols/