[DannyBee]

"is not actual knowledge that the forks are as well"

You assert this as if this is how a judge would see it (or as if it is well tested). That seems highly unlikely to me.

It seems to be very easy to make the legal argument that because github knows which forks are copies and which aren't, they have actual knowledge of the ones that are still copies.

[wtallis]

In order for GitHub to be presumed to know that the forks are also infringing, the takedown notice would have to specify what in the repo is infringing so that GitHub could compare the contents of the forks against the instance that the takedown notice is filed against. If the notice doesn't specify any particular files in the repo, then GitHub can't be expected to remove any fork that has any deletions in its history since the branch. The ease with which the accuser can check the forks for infringement also shifts the burden away from GitHub.