As part of the ongoing shift away from x11 session, KDE removed the x11 session providing only a default wayland session.
KDE plans to have fully removed the x11-session with Plasma 6.8.
As described in numerous announcements and topics you can add an x11 session to Plasma.
Install the package plasma-x11-session
sudo pacman -Syu plasma-x11-session
Log out switch session and log in.