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.
