I do remember when, although I’d forgotten it until you brought it up. Yes, it was part of the audience experience, not addressed to anyone but just coming from yourself as a shared expression of appreciation for what you’ve just seen.
My guess: it faded when everyone became inured to television, so that moviegoing was no longer a special public experience analogous to theatre but more like a magnified private experience analogous to being in your living room.
[Edit] I think the living-room analogy is what makes rude, thoughtless people imagine they have a right to talk. A live performance in a theatre is usually treated with more respect.