I've been looking for info about this for months, as it was obviously a part of the EU's anti-gatekeeping legislation from last year, but I couldn't find any info. Specifically I wanted to know which apps would be able to communicate with WhatsApp - Telegram? Signal? Something else?
And now that there's an article, it's behind a paywall...
Edit: managed to read it through Firefox's reader mode. Unfortunately they don't know, but not for lack of trying:
So far, it is unclear which companies, if any, are planning to connect their services to WhatsApp. WIRED asked 10 owners of messaging or chat services—including Google, Telegram, Viber, and Signal—whether they intend to look at interoperability or had worked with WhatsApp on its plans. The majority of companies didn’t respond to the request for comment. Those that did, Snap and Discord, said they had nothing to add.
The only service they mentioned that definitely will have chat interoperability is Facebook Messenger.... Yeah, no fucking thanks.