As of June 2025, X is no longer installed by default. logging out and trying to select X as desktop instead of wayland does not work since this choice is not there.
To install it do
sudo pacman -Syu plasma-x11-session
Now logout and on the login screen the choice of X11 plasma will be there in the small menu on the lower corner of the screen.
I needed to run on X11, since the appliation peek, which captures animation gif file from screen only works on X11. And all others I tried that do not need X11, were not good at all. On windows I used to use LICEcap for making animation gif files. But this is not available on Linux.