Maybe Usenet would be more of a headache than it's worth, but it seems perfect for what you're describing.
Usenet still exists. Outside very small pools, it's dead, or a spam wasteland.
Peter da Silva, an early heavy user (the first and fourth most prolific poster one month in the late 1980s / early 1990s, according to a reply he made to me at G+), created, and still runs, Usenet II. Which is also a wasteland.
The client-independence is a benefit only up to a certain degree. Consistent characterset (8-bit ASCII largely), no binaries / multimedia (uuencoded excepted), and consistent forms of address and reply (mostly), kept things sane.
But even between the tin/rtin and emacs newsreader camp, quoting styles differed. I don't recall if it was @gumby or others (and I'm positive one case was someone else, involved in xemacs development), but even one emacs-variant style of reply-quoting turned disruptive when trying to nest multiple levels deep.
("Doc, it hurts when I do this." "Don't do that then.")
These aren't insurmountable obstacles. But they are obstacles to be mounted. On which deciding is likely the hardest part.
It's also very slow. And the posting format is very hard to follow sometimes, especially when you get people that don't do hard line breaks (or the other way around, depending on the program you're using) or have funny ideas about quoting.
Also, the lack of moderation seems to create a very wild-west style of conversation, where apparently rational people can start spewing really hardcore invective at the drop of a hat.
Edit: I suspect something like this probably existed. It seems straightforward and obvious enough that I'd bet it did exist, but couldn't get much usage in a world where spam wasn't quite as bad, numbers of users were smaller, and people were already shifting to HTML based platforms.
People in tech undervalue what older communities offer. If the discussion group hasn't become a wasteland after a few years, there is something there.
I'd argue its a usenet/irc hybrid.