You don't employ a team of SRE's with the sole day-job of git maintenance though, your existing SRE/sysadmin team manages it.
And any large business will already be doing Backups, DR etc for their existing systems.
And any large business will already be doing Backups, DR etc for their existing systems.
Any? Are you sure?
If you work in a large business that needs git but doesn't have backups and has no disaster recovery plan at all you're going to have much bigger things to worry about than a git repo being offline.