News outlets have been reporting on how the Terrorgram, O9A, 764, and murder loving European fascist groups have been organizing on Telegram (it's in the name!) for years.
Discord at least attempts to do some moderation about this.
Telegram's strategy to deal with violent extremist groups on its platform is to not ban them but just make it hard to discover. But this doesn't stop someone already in the groups from inviting their buddies. I bet they actually prefer it this way.