They still exist. A small corner of the French free software community uses "tribunes" to communicate, and that was developed a bit before 2000. It consists of a web interface (originally not Ajax) and a standardised protocol supported by a handful of clients (most of which can federate multiple tribunes). It was briefly popular outside this community when I wrote a Drupal module that provided a tribune for any Drupal site, but then the modern web and smartphones happened and today it's really confidential.

I still use it though, and the graphical client running on my system (which is also an RSS reader) uses 23 MB of RAM at the moment, displaying 1375 messages.