You can’t in Wayland.
KWin is a Wayland compositor, that needs the GPU to function. This is very lightweight on the GPU and cheaper (i.e more performant, less energy consuming) to run graphics (aka compositing) on the GPU than on CPU.
I am betting the reason why you want to disable compositing under X11 don’t apply in KWin Wayland.
You just updated to Plasma 6.0 according to the photo of your screen, and Plasma 6.0 by default uses a Wayland session. You can pick X11 session while in sddm.