Does someone know why the counter for this issue is at "70", but the repo only shows "21" issues in total? Maybe there are deleted issues that are not part of the total counter?
No, that's because GitHub uses the same number for both issues and pull requests. We did about 70 pull requests. You can verify that yourself by going on an issue and incrementing/decrementing the number until you hit a PR at which point you'll be redirected.