How to delete unreachable commits on GitHub

The dangling¹ commit shown in the screenshot below should have never been on GitHub, at least not publicly visible. The commit was part of a branch called draft that I pushed to the repository when it was still private. If I am not mistaken then I removed the file, squashed and merged the draft branch into master, deleted the branch and then pushed it onto GitHub. After making sure that branch draft was gone on GitHub I made the repository public.

As you can clearly see the commit is still there, and I only discovered this by accident when using GitHub's search. So, the question is now how to delete this commit? I cannot even reach the commit from my local repository:

$ git show b2444ca54c74a7c9dde6504cf028b6c0f4729d8f
fatal: bad object b2444ca54c74a7c9dde6504cf028b6c0f4729d8f

Its parent commit is reachable though:

$ git show 01aea0c8e1bdceb7869403814a4ebb87e25883b2
commit 01aea0c8e1bdceb7869403814a4ebb87e25883b2
Author: Andreas Linz <klingt.net@gmail.com>
Date:   Sun Jul 25 16:18:10 2021 +0200

    Initial commit

If the commit would still be part of my local repository I could have removed it with this call and subsequent git push.

$ git reflog expire --expire-unreachable=all

But, nothing happens since the commit is not in my local repository. What to do now 🤔? As far as I see there is nothing I can do locally and cloning the repository will also omit the commit I want to delete. GitHub must trigger the garbage collection for this repository so I opened a ticket with them. What really amazed me was how fast they responded, after 10 minutes everything was done and that's for a free account! Here's what they wrote in the response:

As you mentioned, it does indeed appear to be an an unreferenced commit dangling in our cache. I went ahead and performed the garbage collection and cache clearance operations for the repository klingtnet/klingtnet.github.io. Attempting to access the commit directly now returns a 404.

Luckily the commit contained no secrets, but what I learned from this lesson is that I need to double or triple check the contents of my repository before making it public on GitHub. A simple deletion of a branch might not be sufficient.