Slackware Live can be used as a repair disk but the best way I've found is download whatever package you need and then to "Ctrl-Alt-F1" from the login screen and do a chroot to the system you wish to fix, where you can "upgradepkg --install-new".
In Slackware Live I just
Code:
mkdir /mnt/LOOP
mount /dev/sdfoo /mnt/LOOP
and if needed
mount /dev/sdfoo2 /mnt/LOOP/boot
mount /dev/sdfoo3 /mnt/LOOP/boot/efi
then
mount -o bind /dev /mnt/LOOP/dev
mount -o bind /proc /mnt/LOOP/proc
mount -o bind /sys /mnt/LOOP/sys
and
chroot /mnt/LOOP /bin/bash
and you're in!
I don't find any problems with the "age" of the installer. It's excellent and customizable. My newest gear isn't the very newest as of today but just a few weeks ago a Z490 system with an i5-10600K was bleeding edge. It runs just fine with both 14.2 and Current.
As for your specific question to sort out what's going on with plasma login/logout, exactly what did you do from Slackware Live to "fix" your hard install?