MarsSeed [1] filed a deletion request for usd21 [2]: Broken, long unbuildable legacy version. Current version of usd has been adopted to [extra] by @Svenstaro and @alerque. Only that version has actual downstream dependents, so it's safe to delete all old version usd packages from AUR.
[1] https://aur.archlinux.org/account/MarsSeed/ [2] https://aur.archlinux.org/pkgbase/usd21/
