I've been quite embarrassed to discover that linking to a github issue in another repo will create a back link in the linked issue. Removing the link doesn't undo it either, you need to delete the issue completely.
We just discovered this today. We were rather alarmed to find, under an issue for a public repo, a link to a PR in our private repo.
Fortunately, in incognito mode it disappeared, and also when logged in under a different account. So it's not visible to everybody. But for a moment it was a very unpleasant surprise.
"https://github.com./a/b" or "https://www.github.com/a/b" also work as "workarounds". But it is incredibly stupid & annoying that it doesn't even go away after editing it out or even deleting the comment that contained the link.
It's fine, it doesn't ping anybody. Unless you're doing something embarrassing or spammy, I doubt anyone cares. I like being able to see how an issue affects other projects, that can be useful information.
To me it just seems like noise when a "large" issue on a big project references a issue on my personal project with a single user. Having to scroll trough dozens of references would probably be annoying.
If you really, really want to avoid it I'm pretty sure you could use a link shortener (bit.ly etc) behind markdown, and it would mask it (haven't tested)
But please only do this for personal repos where you're the only contributor because otherwise this looks incredibly sus for anyone to click on
I think I stumbled on togithub.com, which simply does a 302 redirect to github.com, which keeps the url understandable and doesn't create the backlink. But I have no idea who actually runs the servers, so I instead went back to a text file.
It's only personal repos, that's why I don't want to create the irrelevant noise for lots of other people.
I wish I could turn this feature off.