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.
Related work 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.
Project page (NLnet): Delta Tauri, WebXDC.