> it's the default expectation that in public space people can be spoken to

It is not the social norm that anyone can be spoken to in public at any time, you are oversimplifying things. E.g. it is largely considered socially inappropriate to strike up a conversation with a stranger on public transit when you’re squeezed in like sardines; we don’t talk to each other to give the illusion of privacy and space. It’s also not considered socially acceptable to have a conversation with a stranger standing at a urinal. There are significant social rules about which adults and children can speak to each other in social spaces. Etc etc

There are and always have been situations where it is more or less socially acceptable to speak to a stranger in public. Headphones is not a new one, I knew in the 90s that headphones meant “don’t talk to me”.