For the record, turns out there was a bit of misunderstanding. 

I used the term deprecate above to mean "warn users (through documentation) that they should not use (a feature)" and not in its Python-dev sense of "remove (a feature) after a period of warning".

I do not think the possibility to mix threading and multiprocessing together should be somehow forcibly disabled. 

Anyway, since my view does not seem to resonate with core developers I I'll give it a rest for now.