How can I tell when a private repo was changed to public? And by whom? Is there any way of knowing who authorised this change?
Do you have a GitHub Organisation? If so, you can review the audit log filtered by the repository in question and it might show who made the changes there.
For non-organisation repositories, there doesn’t appear to be a way to track these changes. I just tested it out on a repository in my own account and there was no log of me making it public or setting it back to private that I could find.
To follow up on @matthewheath’s response, if you’re trying to view information that you can’t find via the audit logs mentioned, you can always reach out to private support at firstname.lastname@example.org and they may be able to help further by digging into your account details, per your request, a little more.