Metacity does not use GTK+ themes, it uses its own theme scheme (/usr/share/themes/xxxx/metacity-{1,2,3}). Mutter is mostly a compositor with WM capabilities, and uses Metacity as fallback for applications that do not use CSD (Client Side Decorations).
Most of Xfce4 components will use CSD for 4.16 release.
Metacity in addition to supporting old xml themes, it also supports themes in gtk.css (I tried using gnome-session-flashback and it supported gtk themes without metacity folder, since gtk 3.2)
Mutter is mostly a compositor with WM capabilities, and uses Metacity as fallback for applications that do not use CSD (Client Side Decorations).
No, mutter and metacity are two different code bases, mutter does not use “metaticy as fallback” for anything.
mutter uses gtk for server-side decorations.
Most of Xfce4 components will use CSD for 4.16 release.
What really matters are applications people actually use. But anyhow, any of mutter, metacity or xfwm4 can deal with CSD and SSD just fine.
Metacity in addition to supporting old xml themes, it also supports themes in gtk.css (I tried using gnome-session-flashback and it supported gtk themes without metacity folder, since gtk 3.2)
Re-implementing decorations with gtk in xfwm4 would be a major work, and if I ever had to undergo such that change, I would drop support for current pixmap themes, just like mutter dropped customs themes. I am not sure people would like that though.