Why setting object that is undergoing deallocation to weak property results in crash
It's not a bug: it's obviously very intentional. It is a deviation from the spec, but it's an intentional one.
Based on the warning, it sounds like they wanted to make it easier to diagnose over-release scenarios, and catching objects that are being deallocated at the time might just be a side effect of that main goal.
They might also consider that, if you're trying to weakify self while being deallocated anyway, and you're not checking for a nil weakref (quite common - lots of block code repeatedly calls through a weakref that could go nil at any time!), you're setting yourself up for hard to debug bugs.
All that said, I'd love to see the notes behind that runtime change.