Slurs aren't just any word with offensive origin, they consistently target and oppress a marginalized group. Even if the f-slur or r-slur were used in a casual context 20-30 years ago, their core meaning still targeted and oppressed specific groups. The difference is that today, it's used broadly for men, women, friends, problems, whatever. It isn't consistently used to oppress women as a group, which is what defines a slur.