i don’t have anything in
~/.local/share/plasmasessionrestorenot even the folder…
Ok, thanks for confirming. So it seems a left-over from KDE 5.
I already tried switching to ‘start with empty session’, rebooting and turning back to ‘last session’. I kinda works, but obviously it works different now. At least Chrome-based software isn’t covered any more, so Signal or Brave windows are not restored. This seems to be a shortcoming of the current implementation, so I think my investigations are finished for now.
Thanks to all who helped to solve this problem 