@tfb also not addressed in my post but since you brought it up, i do think its worth noting that i have so far seen zero evidence of people assuming public posts are not public or needing to be educated about the fact that random people might stumble across their posts. it is true some new people don’t know they have a choice about posting privacy and that may impact their behavior somewhat?
but like, for many people their first experience is making a post that reads ‘hi im new’ and getting several responses from absolute strangers welcoming them to fedi (i know because im often one of those strangers!) and also because i see a lot of ‘oh wow i just joined and everyone’s so friendly here????’ posts