I would say that would not be done by "an app", nor "a service with an API exposed", but by "a protocol" that other/most/all apps follow. Jabber/XMPP comes to m

I would say that would not be done by "an app", nor "a service with an API exposed", but by "a protocol" that other/most/all apps follow. Jabber/XMPP comes to mind, so does Matrix.