MarsSeed [1] filed a deletion request for shmupmametgm-git [2]:

This is a now deprecated VCS package for a customized MAME emulator fork. Code development discontinued in 2020.

Its GitHub repo is inactivated (archived), so it's read-only and even the issue tracker is frozen in time. There are no continuation forks available.

Safe to delete. Nothing requires this package.

[1] https://aur.archlinux.org/account/MarsSeed/ [2] https://aur.archlinux.org/pkgbase/shmupmametgm-git/