It’s not quite that straightforward. There were massive battles between football fans over a hundred years ago. Millwall v West Ham for example was a big rivalry between dockers north and south of the Thames. It started pre 1900, but it’s a small part of history that tends to be forgotten now. I think the ruling classes and newspapers took working class violence more for granted then. However, it’s probably true that it was more civilised between the end of WW1 and the 1960s. Perhaps the experience of real wars took the edge off the male adolescent tendency to gang warfare.
I wonder if there are any books out there about football violence from 1880 onwards?