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I believe you still have a right to use it under copyright law since the license isn't terminated, it's just that potentially you may be infringing some patents, but so far nobody has actually pointed out what those patents would be, so I think it's a valid question.

That's not true: https://code.facebook.com/pages/850928938376556

Facebook can't take away your rights to use React under the BSD license, no matter what you do.

You only lose the patent license, not the copyright license. So it is important to know if facebook actually owns patents covering react and if those patents cover other frameworks as well (e.g. because they cover all virtual DOMs).