Chazza [1] filed a deletion request for gnome-icon-theme-xfce [2]:

Xfce now looks for freedesktop.org icon names for the mail reader launcher and action buttons so this package is no longer required.

Mail reader: https://git.xfce.org/xfce/exo/commit/?id=29474fe37267266e785511808ff2586bd19...

Action buttons: https://bugzilla.xfce.org/show_bug.cgi?id=11513

[1] https://aur.archlinux.org/account/Chazza/ [2] https://aur.archlinux.org/pkgbase/gnome-icon-theme-xfce/