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Open source licenses give anyone the right to use, inspect, modify, redistribute, and distribute modifications of the software. A "fork" is a new unique variant of the software started from the original that diverges from the original to become its own thing. WordPress is itself a fork of b2/cafelog

The framing is false. Nobody mentioned was planning a fork and some of the named contributors haven't had anything to do with WordPress in half a decade (in at least one case due to this very same style of harassment).

This is such a bad reporting. Mullenweg's blogpost is not a factual allegation of fork, it's a sarcastic (and a little childish) clapback at people who criticize his leadership. He's (correctly) implying that if they don't like it, they should just fork WP instead of whining and grandstanding.