Delta Chat is a cross-platform messenger that uses email as its transport. You sign up with an email address — or a dedicated chatmail profile — instead of a phone number. Messages are end-to-end encrypted with Autocrypt and OpenPGP; contacts who already use encrypted mail can chat without installing a second identity system.
NGI-funded work around Delta Chat includes Delta Tauri (a lighter desktop client based on Tauri instead of Electron), WebXDC (small apps that travel inside a chat without extra logins), and related bots and Ubuntu Touch clients. Metadata still depends on the mail or chatmail operator you choose, much as an XMPP homeserver does.