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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.
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.