I personally consider IRC (or I guess more precisely, IRC clients like Weechat (Not to be confused with China's WeChat)) a contender, as it provides chat functionality as well as the ability to create channels for themed discussions.
No, it doesn't have emojis or inline multimedia, but IMO it delivers the important part of chat and leaves the unimportant parts for other applications (like opening a hyperlink from IRC chat to see the clever meme someone posted). It also doesn't do video or voice, but there are other applications for that as well. For purely text chat, IRC is a lightweight (and self-hostable FOSS) solution.
No, it doesn't have emojis or inline multimedia, but IMO it delivers the important part of chat and leaves the unimportant parts for other applications (like opening a hyperlink from IRC chat to see the clever meme someone posted). It also doesn't do video or voice, but there are other applications for that as well. For purely text chat, IRC is a lightweight (and self-hostable FOSS) solution.