The Signal messenger and protocol.

I'm more interested in matrix having worked with meta to actually bridge from a matrix home server to WhatsApp. Thanks to the EU's DMA WhatsApp were required to do something.

Element has been working with Meta since the end of last year to help test their DMA interoperability (given we’re probably the world leader in interoperable end-to-end-encrypted communication) - and Matrix announced last month at FOSDEM that Element has successfully integrated 1:1 chats between Matrix and WhatsApp via the DMA APIs, while maintaining end-to-end encryption (having implemented full Signal compatibility in vodozemac). We’ve also formally requested interoperability with WhatsApp, as of yesterday. https://element.io/blog/the-eu-digital-markets-act-is-here/ (That full post is quite a long read and some of the technical bits went way over my head as I'm more focussed on dev-ops than the inner workings of the protocol)

As for how it will look, further down that same post there is the following:

The biggest concern right now is around “reachability”: whether DMA interoperability will default to off or on for EU citizens - and so whether users on Element would even be able to contact users on WhatsApp without the WhatsApp user having to explicitly opt in in advance. According to the public information available at WhatsApp Chats Will Soon Work With Other Encrypted Messaging Apps | WIRED on Feb 6th: “WhatsApp users who opt in will see messages from other apps in a separate section at the top of their inbox.”. We all know the power of defaults, especially when applied to competition law, and we’re interested to see what the final user experience is.

So sadly it looks as though you may be able to contact people if they've opted in for cross platform messaging, but they'll need to look somewhere else for your messages.

I currently use matrix and have bridges for discord, WhatsApp, RCS, Google messages and possibly a couple of others running so that I can just chat to whoever I want from one place and not care which platform they're using.