Agreed PTR 175, I also check. Only once have I not found it - on a RYR. The CC was surprised and said that she would bring me one. I was not surprised when she didn't!
I decided to see what would happen and let the flight continue. Naturally, no jacket or further comment. I decided not to complain as that would entail letters on my part and nothing on anyone elses' - CAA etc.
So I don't travel RYR anymore but I check my seat every time on every carrier and, if I again find it missing, I will prevent the departure.
Do I think jackets are helpful? In a low energy ditching - Yes.