I first heard the term probably sometime around 2006-2007. I would say rule 34 stuff is taboo in the sense that it is porn, but how taboo probably depends on exactly what the content is. For example, porn featuring Homer and Marge Simpson is probably seen as less taboo by the average person than let’s say Pokemon porn in which trainers are fucking Pokemon. I’m also guessing OP is young given that the story is about sharing things on the bus. I would guess that age would is related to the level of concerned OP is.

Addition: I looked it up, the term was coined in 2003.