The only time I've checked for a life jacket and not found it - was on a flight where we would be over water about 15 seconds after rotation. Bearing in mind this is a critical time of any flight and that we would be crossing more water on our way home, I told the FA when they were checking seat belts secure.
She seemed entirely disinterested in the fact. No replacement was provided.
I am WELL aware that the utility of such devices is low and the liklihood of a 'good' ditching and departure of pax in the way illustrated on the card = low. BUT I would like to maximise my chances! In the flight in question, had we crashed after rotation, it would have been a low altitude and low speed event and thus more survivable.
I won't bother naming the carrier as some folks would say that I was picking on them,
but the carrier is irrelevant - the jacket should have been replaced.