Backends can do whatever. The git backend knows how to push to git remotes. If you used a different backend, it would know what those backends expect. There aren’t any of these publicly, but Google has one to work with piper.
Google's internal git support for the Google3 monorepo was deprecated/is unsupported in favor of fig, the mercurial/hg based client for Google3.
Microsoft has a custom git client internally which includes a filesystem shim (think: the Windows equivalent of FUSE) since stat-ing monorepo amounts of files doesn't work. Also GitHub's running custom git servers, though their custom database backend is behind a compatibility layer.
Finally, sapling, Meta's git replacement which maintains supports for git servers also supports sapling servers (which was not released). Unsure whether to count that as a git backend though.
