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
Simple solution to that is not to add people. Am assuming should that happen Signal will add highlight of sorts to warn about users from other networks.