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

Obsolete. Upstream repo is archived since Oct 1, 2024. Also the used branch, 'gtk-sharp-2-12-branch', has not been maintained for 3 years. [a]

Arch extra repo still carries the last stable release. [b]

[a]: https://github.com/mono/gtk-sharp/tree/gtk-sharp-2-12-branch [b]: https://archlinux.org/packages/?q=gtk-sharp-2

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

Unfortunately it no longer build