In Plasma 6.8, there will be no X11 session in the login screen. There will only be a Wayland session available to log into.
In 6.8, all X11-specific code paths in Plasma for Plasma Shell, System Settings, and device configuration will be gone.
This means that the next distribution releases (Fedora 45, Ubuntu 26.10, and Debian 14 in 2027) will ship KDE plasma with only Wayland.
Our internal metrics within KDE show that over 95% of users of Plasma 6.6 are on Wayland, with a gradual increase every release. The metrics also show that basically no one is testing or developing Plasma on X11 anymore. The platform was already, for all intents and purposes, abandoned by KDE contributors.
This number is most likely sampled by the opt-in telemetry and crash reporting diagnostics. So it could not include those on older plasma releases with X11 or have switched to another desktop for X11.
What do you think? GNOME has already removed X11 from GNOME 50 this year and now KDE is following not that far behind. 
At this point my biggest issues with Wayland are that it doesn't remember and restore window positions and "keep on top" doesn't work for e.g.: Firefox popup video window or mpv. I know KDE said they're going to implement window position saving into KWin eventually but why doesn't "keep on top" work?
There's a workaround in KDE to allow for PiP on top:
It's waiting on the xdg_pip to be merged (https://gitlab.freedesktop.org/wayland/wayland-protocols/-/merge_requests/132) which should be soon (it's hard to tell).
Session restore has been finalized in plasma 6.7 so it's basically a matter of time till apps update to it.