I contacted Github staff from here : https://github.com/contact
Here’s the answer (I couldn’t do anything about it, no prune, no gc, etc)
Hey Maxime,
The commit was available because commits are not automatically deleted
when they’re removed from the history of a branch — they’re deleted
when they’re garbage collected. I just ran garbage collection for that
repository manually and the commit should now return a 404.How often does the garbage collector run on your end?
GitHub doesn’t have a scheduled garbage collection process. We don’t
clear repository caches automatically (we’re in the version control
business, so we don’t delete data unless we absolutely have to 😉) so
usually, the only reason we would do that is if we had someone writing
in to us asking for us to clear them as part of the sensitive data
removal process.It’s also possible we might clear the cache for technical reasons if
the content or structure of the repo was causing us difficulties to
host it, but that would usually only happen if the repo was
exceptionally large or had a wildly structured folder layout.Please let me know if you have any questions about this or anything
else.Hope this helps.
Cheers, XXXXX
So you just have to wait or contact staff to force garbage collector in case you have the same problem !