Interesting. I'm on the site right now and I'm not seeing this. Under Advanced Search, I still see only Economy, Business, First. I also don't see this option under Travelers. Using Firefox. Maybe things are rolled out differently by browser (unlikely, but it can happen)?
Cheers,
I think it depends on combinations of from/to airport.
i.e. LAX-HKG shows Premium Economy, but LAX-BKK doesn't show Premium Economy.
I noticed the change recently, too.