Since nothing is ever really deleted in a repo
Not quite. The git reflog
(for 90 days by default) or git fsck
commands can help you list old commits, but you would still have to remember the branch name in order to restore it (as in here).
And that supposes a direct access to the common repository where those 100 contributors are pushing to.
If the remote repository is managed by a repository hosting service, like for instance GitHub, you would use the Events API in order to find back the trace of deleted branches.
In both instances though, once a branch is deleted, it is not easy to find it back, and that would be even trickier for all past deleted branch.