I went through my private repos and the only one I found was one where I had forked a project, called `documentation`, and my fork made it into the Software Heritage archive. Sometime between then and now I deleted my fork and cloned a private repo with the same name. I confirmed the Software Heritage archive only has the one that was public and not the private one. I feel like we'd be hearing much louder alarm bells if the Software Heritage archive included private code given how many people rely on private GitHub repos.
https://news.ycombinator.com/item?id=39770776