So, I am wondering @joekamprad, reading your comment and igorrr’s solution, is this the preferred method? Just let SDDM be and move to plasmalogin? If so, should there maybe be an announcement?
I will have look on the weekend i said 
No pressure, everything works, noone is in danger 
You saved me, thank you very much
what configfile and where?
Just follow the instructions for a permanent fix here: KDE/Plasma X11 session removed from archrepos / Plasma moved to wayland only
(post deleted by author)