Yep, I remember that topic and seeing it as early hints that Discourse was considering this dev direction. It’s an interesting idea, to be sure. Though it doesn’t necessarily address the UI/UX of it. Currently sidebar/popup seems to be the best “live” representation. How it gets shown to someone coming to the topic for the first time is more of an interesting question to me. And also of course whether chat-on-topic is even allowed at all.
Maybe the suggestion is that chat-on-topic would not be a manual option, but something that just auto-instantiates once a certain reply rate is reached in a Topic?