Probably when the forums just started back in 2012-14 but now it's rarely ever used. Basically it was the Discord before Discord.
Last time I used it (which was a while ago, I don’t like sharing my IP), it was a sort of clunky text messaging type of thing, I believe it’s been reworked since then though.
When Utah got released (i don't remember when), i used the chat app in game (that is the same as the forum's chat i think) and i could talk with tdev and nadeox , idk why they where there but it was funny