this might fit better on the wayland page, as SDDM is not exclusively for KDE

How's that? First time setting up void successfully, so I'm still not super familiar with runit.

the existence of a file called down in a service means runit will not start it, even if enabled (symlinked)

If the file service/down exists, runsv does not start ./run immediately. The control interface (see below) can be used to start the service and to give other commands to runsv.

https://man.voidlinux.org/runsv.8

I think it makes more sense to frame it something like this, so we don't rely on archwiki:

To run SDDM under Wayland, install weston, set DisplayServer to wayland in sddm.conf(5), and disable agetty-tty1 ...

also, no need to mention xorg because that's not relevant in the wayland page

last thing, commit message should be something like config/graphical-session/wayland: explain running SDDM on wayland

Are you folks interested in merging, or should I close this?