MarsSeed [1] filed a deletion request for usd21 [2]: Broken, orphaned legacy version of usd, cannot be built.
AUR has the functioning latest version: https://aur.archlinux.org/packages/usd Nothing depends on this old one. [1] https://aur.archlinux.org/account/MarsSeed/ [2] https://aur.archlinux.org/pkgbase/usd21/
