You seem to have some serious reading comprehension issues. The context of that phrase was this sentence: "The point is that breaking legacy code doesn't have

You seem to have some serious reading comprehension issues. The context of that phrase was this sentence: "The point is that breaking legacy code doesn't have to be a show-stopper for eliminating the GIL, and backwards-compatibility does not have to be (indeed should not be) an overriding consideration in a GIL-free Python." That has nothing to do with opt-in vs opt-out.