Over the years there's been a couple of apps that have tried to use email protocols as the backend for chat. I really wish those had gained popularity - there's a lot of overlap with messaging and email.
DeltaChat supposedly does that but I've never tried to interoperate it with email
Isn't that just email then? I mean I guess you could wrap a bubbly UI around it, but you're not getting around the latency and spam. Those seem like dealbreakers to me.