If you know the hash of some data, then you either already have the data yourself, or you learned the hash from someone who had the data.
If you already have the data, there is no vulnerability - since you cannot learn anything you don't already have.
If you got the hash from someone, you could likewise have gotten the data from them.
People do need to be aware that 'some random hex string' in fact is the irrevocable key to all the data behind that hash - but that's kinda inherent to gits design. Just like I don't tell everyone here on HN my login password - the password itself isn't sensitive, but both of us know it accesses other things that are.
If github itself was leaking the hash of deleted data, or my plaintext password, then that would be a vulnerability.