Thanks for the report.  I just reviewed xinitrc, and this is the culprit:

# FIXME: This should move to /usr/bin sometime, with a backward compatible
# symlink added back just prior to release, to help people to fix things to
# use /usr/bin instead.  Then keep the symlink for a release or so.  When
# this changes, X11BINDIR in the Makefile needs to be updated also.
/usr/X11R6/bin/RunWM

RunWM is an archaic script that I don't think anything actually uses anymore.
I left it around, just in case, however I think it is safe to kill it now.

The xorg-x11-xinit package obsoletes 'xinitrc' in the modular tree, and does
not contain the RunWM script.  I've moved all of xinitrc into the new
package, and also updated all paths pointing to X11R6 in the scripts
themselves.

I think we're good to go for this one, even though current rawhide xinitrc
still has the RunWM script, but that's ok.

I'll leave this one open until we push modular X into rawhide.

xinitrc is gone in the modular X staging tree