The stoplight indicator and a few other things were done via client side js with non obvious variables names. Once that Greasemonkey script came out it was quickly moved to the server side since there was no reason not to do it there. I believe it was just on the client side due to ease of development when it was being built.
You could see how long ago the person recieved their last message, how many messages they were getting on average and also I think some scale thing which could extrapolate how frequently they wrote back to messages. It was a long time ago but incredibly interesting / invasive.