Short answer
Long answer
I had a try on a test Arch-plasma install.
Before upgrading I removed plasma-wayland-session and all its dependencies.
I logged out and as expected I had no Wayland option in the login screen.
I did the upgrade to plasma 6.
After reboot, Wayland session was the default login session with X11 as option in the dropdown menu at the bottom left of the login screen.
PS.
I had a test Arch-plasma install, because: My Every New Year’s Resolution 