it scaled as well as you would expect coming from those days.

the alternative was a whole computer plus modem per user.

SSE is cool, but as others have pointed out, this isn't that. I also wouldn't call this Comet: almost unilaterally, Comet referred to using it in concert with JS/Ajax and usually was just a fancy way of saying long-polling (though I admit it's a bit broader than that in truth). What CGI:IRC did was different: it streamed HTML directly. More to the point, it didn't use chunked encoding or any kind of framing in the body. It just kept the connection open and kept sending HTML in realtime. I don't think this ever had a term associated with it, or at least I was never aware of one.

SSE came along much later. At the time it was common-ish to just slowly send HTML tags. I think at some point there was a gorgeous hack where they dynamically rendered and sent GIF frames with the messages of the chatroom.

The broader technique became known as Comet: https://en.wikipedia.org/wiki/Comet_(programming)

CGI:IRC used an early frame approach, no JS required.

At some point JSONP became popular, even for 3rd party APIs.

  <script src="http://dont-xss-me/please?format=jsonp&callback=lol">

Server writes:

  lol({ ... })

  lol({ ... })

That’s the modern variation. The original didn’t have any framing like they.