They can't use this function, but with some duplicated unsafe code they can still provide similar APIs for their callers, right? Things like this: https://github.com/jaboatman/tokio-scoped
That said, taking a step back it has crossed my mind that bringing back true scoping with unsafe in the impl might be an acceptable way forward, despite how upsetting this is to a lot of rustaceans. I think that the failure modes (ie triggering UB) for this case can be simply enumerated in a short list of "DON'Ts" (such as putting a scoped join handle in an Arc).
OTOH, since leaking was formalized and explicitly allowed (heck, even borderline encouraged through mem::forget), god knows if people have exploited that for other cases in true Hyrum's law fashion, that breaks the abovementioned idea. They certainly had the right to.