i'm not even sure about that. i think i remember the story about islamic terrorists in europe last year using plain old SMS to coordinate their attack. didn't make a difference though, their communication was only analyzed after it happened.

so for the average imbecile just using whatsapp, signal or a self-hosted service would actually make a difference, but if it'd make a difference for law enforcement is a different matter. having access to billions of messages and actually doing something with them are two different kinds of beasts.