Chat online using Bitcoin addresses?
The idea was around for some time, and as Pieter mentioned, using addresses without any additional obfuscating seems like a bad implementation.
There are few other projects, the one that is most serious is Microsoft's ION decentralized identity project. It's not the chat-app itself, rather the way of signing into apps using bitcoin-generated identity.
When it comes to chat apps, there is a pretty rapid development in the field of lightning network. As messages on LN are onion-routed, it seem to be a good idea for privacy-preserving communication.
There are two projects I heard about, self-custodial Juggernaut and centralized (so what's the point) Sphinx.
I'd be very hesitant in using these solutions, they are working as untested, beta-version projects, so they may not be as stable and secure.