Usenet was a decentralized message board system, kinda similar to email. That means all of the discussions that took place here are for the most part, lost to the sands of time. There's no centralized servers in USENET, its all decentralized.
Google Groups became one of the more popular USENET services in the early 2000s. But the archives of USENET posts are likely incomplete. Maybe someone with more knowledge of this early internet would be able to comment on where Google got their archives from.
More info here: https://en.wikipedia.org/wiki/Usenet