It's dangerous enough (suffocation, freezing) that there are plenty of reasons to require an age minimum. I don't think I've seen dry-ice in supermarkets since COVID started, but I haven't been looking much either.
Never disappeared around here. A lot of people rely on it to get their frozen stuff home without thawing out. So it's more available in rural places where people will have a long drive home.
Ahhh, that makes a ton more sense now. As one of the aforementioned soda-bottle-bomb-making-youngsters, I had no idea what people's legitimate uses for it were outside of party tricks.