These thinner seats are infact included in all the airbuses. They were retro fitted into the ET sections of all the original ones.

The difference with these new planes is that they also put them in at the front instead of the convertible seats which have remained in the older planes - hence the feeling of much more leg room.

In fairness if you sit far enough back to be in a row with only 31in instead of 34in - it steill feels better than the older style seats.