The origins of git was Linus sending and receiving patches by email. It is truly serverless.
and also useless as a collaboration tool, now you need email infrastructure up to be able to work. I don't think I ever need email to do my job. Slack can be down to, all I need is github/gitlab
You can viably run git over sneakernet. Yes if you decide to use a specific communication service, that service needs to be up, but git itself is entirely agnostic.
You don't need the internet. Git will work with USBs.