Yes, it was. From their site: "It has been trained on a selection of English language and source code from publicly available sources, including code in public repositories on GitHub."

The issue of deleted repositories being available through it would still exist. Whether or not GitHub should be blamed for that is another matter.

Once you put something on the internet, you should assume it still exists out there somewhere even after deleting it. Even before copilot, all credentials that end up in a repo needed to be changed. I'm not sure what's supposed to be different now.

Yes. nomercy400 is wrong.