Based on my project management notes it was somewhere around June 2014 -- or at least that's the first time I was informed by reservations that Y no longer had last-seat availability when trying to figure out why a Y fare with F less than half full couldn't clear the upgrade immediately.

Also, once that changed -- IIRC because it was a while ago and I'm getting old -- at booking if space were available it would ask you if you wanted the upgrade, otherwise you would have to proactively chase the availability and "request" the upgrade (which itself was a buggy mess when I last tried using it a couple years ago)