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
Also to add a very important question: will Facebook then be able to harvest all of our Signal metadata like they do for WhatsApp?
Hopefully the opposite is true. There’e end-to-end encryption and if the client isn’t written by meta, then meta can’t intercept after decryption.
Being able to use meta’s chat platform without any of their code being executable on your device is a big improvement. They can’t scrape your contacts and data at all.