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China is attempting to mirror the entire GitHub over to their own servers, users report
(infosec.exchange)
This is a most excellent place for technology news and articles.
Does it though? You can still put up a fork somewhere else as long as you uphold the license right? Unless I guess in the case where the license explicitly disallows forks, but I don't think that's very common (can you even do that?).
Forks are derivative works (quite obviously) so yes you can forbid them via license terms. Whether or not that's still open source, take it up with OSI. I vaguely recall that at least once upon a time there was some project that required modification to the code to be published as separate patches and it was generally accepted to be open source don't ask me which.
Most GitHub repos don't have a license, meaning you are not licensed to do anything with them. Rehosting them would be the same as rehosting an image you don't have a license for.