[edit: turns out CSD is auto-fallback on QT Wayland. It's just that nobody has seen it yet due to another bug in Mutter which prevents QT apps from running with or without CSD. QT falls back to empty titlebars in its own style, a behavior that can be disabled using an env variable. I've included a screenshot of a QT app with CSD at bottom, for anyone interested. This is what all of them will look like once the bug gets fixed.\* )
All the major KDE apps have been ported to Wayland. The same is probably true for a bunch of other QT programs. But there is one catch: they won't run on the "default" linux desktop because of Mutter's CSD requirement. CSD, short for "client-side decorations", means the client has to draw its own decorations: everything from the titlebar to the shadows. Currently, the vast majority of Linux applications delegate this pretty hefty task to the window manager.
While Gnome has made CSD work for their apps and their desktop, most non-GTK developers and users don't want anything to do with CSDs on Linux. They are a pain to develop, ugly, inconsistent and make it harder to move your windows around. And users will flood you with bug reports demanding normal-looking decorations. This happened to Opera and VSCode devs - their CSD was completely rejected by users. Firefox on the other hand spent years making their CSD and they still can't figure out how to round the corners on Gnome or draw shadows on KDE. Aside from Chrome, these three are the only important non-GTK applications that have tried anything with CSD, and trust me I've looked.
But to get "normal" decorations on Gnome, you have to give up Wayland, that thing people have spent ten years trying to make happen.
Now we have this all-important-thing (kind of) and Wayland clients are being prevented from running, indefinitely and by design. Gnome requires all Wayland clients to draw their own decorations, with shadows and everything. If the app doesn't do that, it falls back on X. All this dates back to the developers of Wayland, who wanted everything to be CSD (even if technically Wayland doesn't rule server-side decorations out).
Does anyone here think this is anything short of disaster? This problem will continue indefinitely until something changes. Basically, if you use Ubuntu and want Wayland, you better learn be stick to only Gnome apps (except Firefox and Chrome, which have pulled off half-assed CSDs that make it nearly impossible to drag a window without using the alt key).