From my experience on other Princess cruises, they have retained the formal nights, usually two on a 7 day cruise. But they seem to have cut back on the number on longer cruises. For example, on a recent 15 day Panama Canal cruise, we were told there would be 3 formal nights. Once on board, they reduced it to two. After almost a mutiny by the women that packed extra for 3 formal nights, they added it back on. But if they went according to the 2 per week, there would have been 4. Two or 3 is fine for me.
The one thing Princess seems to have done is eliminate the semi-formal nights. I think that was a good idea. I don't mind humoring my wife with a couple of nights to play dress up (I accuse her of never getting over doing that as a child <G>) but having that extra suit or suitcoat and what goes along with it, did take space in the luggage.