If you establish that so-called fascist speech is illegitimate, then who decides who will be targeted as fascist? Can’t it lead more mainstream politics to end up being targeted?
When anti-fascist groups successfully defeat the organizing of local neo-Nazis and fascists, what usually happens is their group falls apart and individuals go back to being labor organizers or environmentalists or whatever kind of leftist. The lifecycles of anti-fascist organizing rise and fall with the organizing of the far right.
Anti-fascists oppose anti-Semitism and Islamophobia and there is a certain political lens that — agree or disagree with the lens — there is an element of continuity in terms of the types of groups targeted. I don’t know of any Democratic party events that have been ‘no platformed’ (shut down) by anti-fascists. So there is a political lens, people will quibble about what the lens is, who designs the lens, but I don’t think the slippery slope is actually, in practice, nearly as much of a concern as people imagine it would be.