1. Applications or toolkits only supporting Wayland. An example of this is Waydroid, though thankfully that's the only example I'm currently aware of.
2. Desktop Environments only supporting Wayland. Examples now include GNOME and KDE.
3. Drivers only supporting Wayland. I think this may be the case of some "exotic" systems; I believe there are some postmarketos systems that don't have graphics with anything else. Thankfully, the existence of Wayback means this is probably a non-concern.