I’m just saying that if you have an EOL kernel its 1 obvious thing that is broken.
There are probably other issues. We cant know without more information.
System info:
inxi -Farz
Pacnews?
pacdiff -o
Unsupported packages?
pacman -Qmq
Orphans?
pacman -Qdtq
Are you really up to date?
sudo pacman-mirrors -f && sudo pacman -Syu
Did you ever refresh desktop cache and configs (especially between major plasma versions)?
KDE - ArchWiki
Do you use 'saved session’s?
Today's upgrade KDE Plasma Logs Out to a Black Screen
What happens if you enter a shutdown command?