IANAL and all the other anals, but my understanding is Signal wouldn't be liable and wouldn't have to do anything. They designed their service so they can't know the content of the messages, so if a third party Maloyse (see what I'm doing there?) is reporting a message between Alice and Bob that Maloyse thinks to be illegal, Signal would be within legal grounds to bring into question how did M got that message, and it can't be used as proof against Signal because there is no legal mechanism by which Signal could have acquired that message and act upon it - in fact, Signal has grounds to suspect Maloyse is crafting those messages, since neither Alice nor Bob have reported such message.