I couldn't find anything about it being dropped either. From SE-0329 (Third review):
- It was observed that the preposition
for: in sleep(for:) could be omitted as a needless word. However, there are good reasons to keep it: most importantly, that this is one method in an overload set where the alternatives ( sleep(until:) ) use a contrasting preposition. The Core Team agrees that for: should remain.