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
Carriers already support it, and I think the reason Google sends it through their backend instead of the carrier’s is so they can support their proprietary extensions (that’s my guess at least, and I think I read that the app can also use carrier services directly but I can’t find the text that said that anymore). And as far as I’m concerned that means there are indeed many phones that support it.