Presumably if you as a user want to use an app that interoperates with Signal, Whatsapp and Messenger then you'll be logging in with your signal, whatsapp and m

Presumably if you as a user want to use an app that interoperates with Signal, Whatsapp and Messenger then you'll be logging in with your signal, whatsapp and messenger credentials and give it permission to read your messages and relay them I don't think this is to be interpreted as "from now I need to be able to send messages from a Whatsapp client to a Signal client" Of course any such app would be able to read all the messages in the clear and would be able to store them in the clear, leak them, sell them or whatever. As with any other case in which you choose to use a chat app you have to trust the chat app to read your messages if it wants to I imagine if that's the case whatsapp or signal, when you do a first login from a different app, will flash a warning that you're using a third party client which might not be trustworthy