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Almost all the code done is by Steemit Inc, witnesses can refuse it by not updating or doing their own fork. As of now, no one outside of Steemit Inc has done forks. There have been pull requests by users and witnesses that have been approved, but most stuff just sits in limbo and never gets touched.

But theoretically a witness could produce a fork and get enough other witnesses to approve? And if so, is it 50% plus one to pass a fork, or is it actually on a consensus model?