Not that much anymore. Riot made a good job at detecting slurs and banning their authors. Racist and homophobic slurs have completely disappeared from the game from what I observed in the recent years.

To compare, I've played some games of CS:GO this week and got called "fils de pute" in one of them. In another one, a guy spammed the n word for 5 minutes before being kicked manually by his team. It's also common to heard people arguing about how they will fuck or kill each other.