Whatsapp problem isn't in the algorithm, but their identity resolution (hence their roundabout way of doing their web client: https://github.com/sigalor/whatsap

Whatsapp problem isn't in the algorithm, but their identity resolution (hence their roundabout way of doing their web client: https://github.com/sigalor/whatsapp-web-reveng https://github.com/sigalor/whatsapp-web-reveng) iMessage approach is to treat every conversation as a group chat between all the recipient devices and all of yours. There's another risk here, that Apple doesn't let the end user verify the list of devices the chat is encrypted for. Which is to say, a perfect E2E experience isn't trivial to design, but you don't have to drop the ball completely.