That’s very easy. Just open the index-v1.json in an editor and search "sourceCode": "https://github.com.
This is what I got.
| repo | count |
|---|---|
| github | 2986 |
| gitlab | 350 |
| google code | 68 |
| codeberg | 62 |
| bitbucket | 56 |
| sourceforge | 50 |
| sourcehub | 4 |
| gitea | 1 |
And some selfhost repos.