That's information disclosure; nothing about the repo after it's privated should be known, including name changes and deletion.

It’s unclear to me what the attack vector is for that disclosure.

It was known and there are many sites scraping GH for metadata that will have it anyway

Not whether or not a repo exists after it's set private.

This information is useless. It leaks nothing but name and existence of something once public (that vanished from public view). Most importantly it's not confidential or critical in any way