Got it from here : https://infosec.exchange/@mattburgess/111885761552024420
NEW: WhatsApp will soon make it possible to chat with people who use other messaging apps. It’s revealed some more details on how that will work.
— Apps will need to sign an agreement with Meta, then connect to its servers.
— Meta wants people to use the Signal Protocol, but also says other encryption protocols can be used if they can meet WhatsApp’s standards
— WhatsApp has been testing with Matrix in recent months, although nothing is agreed yet. Swiss app Threema says it won’t become interoperable
there’s no paywall, the whole article is readable. but in summary, they don’t know yet which apps will have this implemented, if any.
They give you only so many free articles per month, I had to circumvent a paywall as well