There was a point a while ago where there was at least an acknowledgement of it (The Blue Book?) And Billy Boys was all but eradicated (at Ibrox anyway).
Definite resurgence in the past few years, possibly down to the scrapping of the OB@F act?
Loads of whataboutery going about on Twitter etc.
There is a huge common denominator - You don't see much trouble unless certain clubs are involved.
Thousands indulging in sectarianism etc every single week, no matter who they're playing.