On Plasma 6.4 the wayland session will be the only one installed when the users does not manually specify kwin-x11.
With the recent split of kwin into kwin-wayland and kwin-x11, users running the old X11 session needs to manually install plasma-x11-session, or they will not be able to login. Currently pacman is not able to figure out your personal setup, and it wouldn’t be ok to install plasma-x11-session and kwin-x11 for every
one using Plasma.tldr: Install plasma-x11-session if you are still using x11
Does that mean sddm is now wayland or is that still X11?
Still X11. We’ll see what happens when (or if) it gets replaced with KDE’s fork.