JOIN/PART support will be a separate discussion. There will be a thread some time in the future to discuss our thoughts on how to support this feature without it impacting chat for regular users. I don’t want to derail this thread with that discussion though.
Don’t want to derail this thread, but currently we have no plans to support it. We actually do have the infrastructure to send messages to individual users (and it is used for some features), but we’re pretty sure our server cannot scale to the usage if we made the feature available to everyone. Supporting it would be a pretty big project, and we already have a lot on our plate to make the current features work.