Ctrl+Alt+Backspace should always be able to kill curently running session. When wayland session is running and everything including cursor freezes, there is nothing that can be done.
If the cursor is frozen, the process that handles input (and the only process than can forward control+alt+backspace) is frozen. Not much we can do at a KDE level.
(In reply to David Edmundson from comment #1) > If the cursor is frozen, the process that handles input (and the only > process than can forward control+alt+backspace) is frozen. > Not much we can do at a KDE level. Could you create new process just to handle this 'session-killing'? Or some kind of freeze detection that would kill the session when it freezes?
Watchdog on kwin would be viable if/when we ever get that systemd user units port sorted.