X11 applications will still "work" on Plasma Wayland session by utilizing XWayland, a tool that (afaik) runs a compact X11 session for each application.
What is being removed is running an X11-native Plasma session, only Wayland Plasma sessions will be available starting with Plasma 6.8.
What is being removed is running an X11-native Plasma session, only Wayland Plasma sessions will be available starting with Plasma 6.8.