I listed the assumptions that I made to demonstrate the possibility of a scenario. That said, you've grossly simplified the situation. What if it is an unreferenced commit from a year ago. Would you really find it damning that you can't go to Github and say:

  Hey guys,

  I had a branch `foo` in my repository `bar` a year or
  so ago. I deleted the branch (which had no references)
  and don't have any backups of it. Do you think that
  you guys could restore it for me?