The anti-Signal rhetoric has already started. News articles about Antifa specifically mention that they communicate via Signal. If Antifa is designated as a terrorist organization, then we'll see all the counter-terrorism tools brought to bear against them. If the state can't break Signal encryption, then you'll see renewed energy for anti-encryption / anti-privacy policies.
It's impossible to designate Antifa as terrorist organization in practise because it's not an organization. It's a movement. It's also impossible to designate white nationalism or alt-right as terrorist organizations.
If they can name actual organizations and name people participating in the movement, it might work work against them, but they are not Antifa.