Using a git repo has been suggested many times but so far noone tried to implement this. I expect bundling a git client with desktop and especially mobile would not be an easy task, assuming it can be done with React Native.
Not sure about performance either. The unit of operation for Joplin is a file read or write, and the unit for git is the commit. So does that mean there should be millions of commits, one per file? Or if not, then the way Joplin sync works would need to be changed completely.
Then there’s the need to manage ssh keys, passwords, maybe 2FA, etc. I also wonder what will happen when someone rebase or rollback commits, since Joplin is not designed to handle this either.