I'm not sure if/when there was a change in this perk, but there was a similar discussion in this forum about 16 months ago. So this is a not a new thing (or at least not that recent). The discussion then was also that you could get a Y seat only if the entire plane was full.

Part of the reason - I'm guestimating here - may be that if you are given the opportunity to purchase a Y ticket in a full Y cabin there are probably no systems in place to keep you from getting an op-up come flight day if J/F is wide open (or if the current systems can't provide that functionality). It would create an incentive to hunt for already oversold Y cabins in the hope of flying J at the price of Y.