I thin that this misses the point. The fringes and conspiracy theorists grow by catching new people from the mainstream. If you segregate the extremists from the center, you can starve the extremists.
Regardless of the precise definition, it seems quite clear that it ought to include those who organize premeditated violence. This is not exactly a slippery slope.
Just ban people who spout violence towards people for what amounts to the crime of existing, who spout conspiracies that are divorced from reality.
You don’t choose who is extremist, you ban the abusive content. Extremists the mostly post that are functionally banned - that’s what Twitter does. You don’t get to report “this person is abusive” just tweets that have to be abusive on their own.
