The attached patch creates additional session definitions for X and
Wayland that launch Plasma in a locked state.  Using them with
auto-login makes for a great combination.  I had been looking for
something like this for years.

The patch applies against 5.21.5-3, but I have only tested it with
5.20.5-6.

Locked auto-login under X works nicely and it's what I'll be using.
Locked auto-login under Wayland works, too, but is a rather rocky
experience IMO.  I also recommend using throwaway user accounts for
testing (to not mess up your saved session) and running top/htop on a
separate VT during the familiarization phase and keeping an eye on CPU
usage.  Notable observations from my testing:

* kwin_wayland pegs the CPU to ~385% until you unlock the session
  (with DRM backend on nouveau).  I guess disabling some effects could
  reduce that, but I haven't looked into that.

* Consequently mouse movement is choppy, and you have to put the
  pointer directly over the password field for it to retain focus.

* The window dimensions of the kscreenlocker_greet window change once
  Plasma launches the Task Manager leaving a black bar at the position
  where the Task Manager would be.

* Also upon logout sometimes stray processes (korgac,
  kactivitymanager, kded5) stay around which consume ~200 % CPU till
  killed.  I can't reliably reproduce this yet, though, so it might be
  nothing.

Regards,
Dennis Filder