We've been on many birthday cruises but have never seen this. DH received this invitation during our current sailing. About 100 guests showed up including family members. Door prizes and a huge cake were provided. I asked the hotel director about it and he said it is new. Rather nice.
Recently on Radiance class, where the machines are at the back of the Windjammer, at times you could serve yourself and other times a crew member was there to serve you. Haven't been able to serve yourself on any other ships we have sailed lately.