This used to be true, but not anymore. We have upstream support for FreeBSD in libwayland now. Other BSDs have MRs which are only waiting for CI support.
Wayland itself has nothing to do with systemd, many users are running Wayland without systemd.
Yes I can confirm that Wayland runs fine without systemd!
I am on Gentoo and using openrc and sway runs fine. The only thing that might kind of rely on systemd is sway-idle which wants logind (or to run as root iirc).
I know you know this, being the sway and wlroots maintainer and all but I figured I would show a concrete example of this working fine.
FWIW, swayidle only has an optional dependency on logind. It can be built without. In fact, we're even discussing removing the logind-related features from swayidle. (swayidle never required running as root though.)
Wayland itself has nothing to do with systemd, many users are running Wayland without systemd.