According to the brochure, they are on request. But there is no charge and all cabin categories get one. And they are terrific. Plush terrycloth, not that waffle fabric stuff. You can also buy one if you like
Just be aware that the bathrobes are loaners. You can buy one, if you like them (we do!!). My sis-in-law assumed that that were part of our mini-suite package and almost packed hers. Since they'll charge you, if you take one - you might just as well get a new one!! :o)
Until last year or so, Princess automatically provided bathrobes for all passengers as part of the cruise package. In order to reduce laundry, though, the company changed the policy to providing them upon request.
Princess also provided a bowl of fresh fruit to every cabin in the past, but ended up throwing away a lot of fruit that rotted because the passengers did not eat it. Again, it's now provided upon request to cut out the waste.