In this specific case, DMCA doesn't apply. But it's interesting to read GitHub's policy about taking down forks.
> GitHub will not automatically disable forks when disabling a parent repository. This is because forks belong to different users, may have been altered in significant ways, and may be licensed or used in a different way that is protected by the fair-use doctrine. GitHub does not conduct any independent investigation into forks. We expect copyright owners to conduct that investigation and, if they believe that the forks are also infringing, expressly include forks in their takedown notice.
> GitHub will not automatically disable forks when disabling a parent repository. This is because forks belong to different users, may have been altered in significant ways, and may be licensed or used in a different way that is protected by the fair-use doctrine. GitHub does not conduct any independent investigation into forks. We expect copyright owners to conduct that investigation and, if they believe that the forks are also infringing, expressly include forks in their takedown notice.
https://help.github.com/articles/dmca-takedown-policy/