This has always been described as 'experimental'

We don't think this has any users: This mode has been disabled in Cygwin
packages since March 2016. We've never provided the xwinwm WM for x86_64
Cygwin. No one has even asked where the option has gone.

This leaves XQuartz as the only user of the rootless extension.

Remove --enable-windowswm configure option
Remove multiwindowextwm stuff from Makefiles
Remove -mwextwm option
Remove -mwextwm from man-page and help
Un-ifdef XWIN_MULTIWINDOWEXTWM

v2:
Remove rootless include paths
Remove windowswmproto from meson.build