I couldn't find anything about it being dropped either. From SE-0329 (Third review):
- It was observed that the preposition
for:insleep(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 thatfor:should remain.