Rocket Chat supports WebSockets. They don't advertise it very well, but if you look at the nginx config under the manual installation, you'll see that the 'upgrade' header is added when using nginx as a reverse proxy, presumably for ws support. Also, they have an experimental realtime API that uses WS and it looks like it's just the API that is still experimental, while WS are already in use for the regular desktop app and the new (still buggy) mobile app.