On my computer, I have a high pixel density screen. Due to some display problems, I chose not to change the scale, but rather the display resolution. The resolution of the greeter does not match the resolution of the Plasma Shell. I think it could be interesting to add this feature especially since, although I say it in a non-expert way, I think it would solve some bugs (1). This feature could be put in the kcm sddm parameter synchronization function. (1) (notably a bug that moves the Plasmoids at each session opening in Plasma 5.20.4 and for which I would put the link below when I would have reported it here)
The bug that I think could be avoided thanks to this proposal is this one: bug 431432
This is out of scope