@menelion Even in v9, that change would break any existing in_array() call where the dev is relying on non-strict behavior. It's impossible to know how many hundreds of thousands of lines of code that is.
I'm a strict typing fan, so I have no use for false there. But IMO the answer is to move away from arrays and build a proper collection trio that is strict by default. (Sequence, Set, and Map, as separate things.)