No. V8 has moved to Chromium's Git infrastructure, not GitHub.

https://chromium.googlesource.com/v8/v8.git

I don't know about them moving to Github, but the v8/v8 repo [0] (currently the svn mirror) has been deprecated and the git repo mirror will replace it in the near future [1]. That's the extend of them using Github as far as I know.

[0]: https://github.com/v8/v8

[1]: https://groups.google.com/forum/#!topic/v8-users/VgL3Fn4ijqE