There can be a lot of hypotheticals. GitHub does not preclude you from using git for what it’s intended to be - a distributed source control management system. If you don’t have backups, that’s on you.
https://sourcehut.org/blog/2020-07-03-how-we-monitor-our-ser...
https://drewdevault.com/2019/01/13/Backups-and-redundancy-at...
When your mail server goes down, nothing happens, the internet continues to function normally and then when it comes back up you will receive all of the emails that were sent to you when it was offline. If the mailing list goes down, you can still send emails and you're collagues probably won't even notice the outage because they were CC'd. When the mailing list comes back up, it'll receive all of those emails in the same way and get caught back up.
When GitHub goes down, everything stops. And GitHub goes down often!