Twilio has a WhatsApp integration. It's paid per message, but it's pretty cheap if you're just forwarding a single user's messages. Probably a bit expensive to do at scale though.

I'm not sure what kind of open source apps leverage it, but I would guess there is something.

Matrix?

Is matrix the other Im service or the tool with which you can forward WhatsApp messages or both?

Matrix is the other IM service. This is the tool for getting whatsapp chats into matrix: https://github.com/tulir/mautrix-whatsapp

Wait until EU gets their data portability laws ironed out.

Yeah, I mean they really ironed cookies out something fierce, with every website now spamming us multiple times a week probably asking if we want to store cookies. So ironed out.

Websites that do that are actually in breach of the regulation.

The problem is that they're allowed to get away with it and the "4% of global turnover" fines are yet to be seen.