Instead of heavily limiting account creation, Discord for example limits the possibility to message users outside of your network by default. Only people you have added as friend or you share a server with are allowed to message you by default.
For signal that would be harder to implement since it's more focused on 1o1 chats instead of groups, maybe if spam gets out of hand they could use a grey-listing approach like Instagram does where users outside your network get moved to the "message requests" inbox by default.
For signal that would be harder to implement since it's more focused on 1o1 chats instead of groups, maybe if spam gets out of hand they could use a grey-listing approach like Instagram does where users outside your network get moved to the "message requests" inbox by default.