Matrix. With its bridges you can "wire-in" networks like WhatsApp, Slack, Signal, Telegram, Discord, iMessage, SMS, e-mail, ... and have a single app that interacts with them all. You can have a single group chat with users from all those networks participating and no one would be any the wiser.
This is true but if you're self-hosting it's not that much bother to add additional copies of a bridge for other users (granted, it's not ideal).