It's a dead script that was not that common in the first place, in Kievan Rus' it was even used as a form of encryption in XI—XVI centuries for how little spread it was. It is also very different from modern Cyrillic. So, saying "most Slavs don't know how to read it" is a bit of an understatement. Noone knows how to read it, apart from some linguists and overzealous Witcher fans.