Unfortunately doesn't seem like many, as most apps use some sort of toolkit, and generally most support either only X11 or X11 and Wayland, supporting only Wayland isn't popular.

Though people who want to avoid using toolkits would probably do that, as Wayland's API is much more sane for apps.

However for example Waydroid only supports running on Wayland, and somehow nobody has created a reverse XWayland yet (People that ask for this get constantly redirected to nested compositors which is absolutely the wrong thing, a proper reverse XWayland would seemlessly integrate the apps like Xwayland does, and you could drag them, transparency would work, and https://wayland.app/protocols/xdg-shell#xdg_surface:request:... would be converted to _GTK_FRAME_EXTENTS, etc)

I don't remember which anymore, but a bit over a year ago, I tried switching everything to Wayland, and had too many problems in my specific hardware setup, went back to x11 and some things just broke and some apps I couldn't use. It was not a pleasant journey, so to say. The transition itself can also be a problem, some things can stay in a config and then not be properly reconfigured.

I've used sway, as an example to Wayland only, and if you have specific things configured around Wayland as a compositor, they break too..

But that shouldn't really be the focus point of the discussion, what really is the point, you don't just yank a whole system wide config out from under an unsuspecting user, there's so many variables you just don't know or can account for. Creating a stable application, is also respecting other apps and the system it runs on, not just going in blazin' fixing things for yourself and then not caring about what side effect/consequences it can have. And worse, not informing about it properly.