An answer is good, Rob. The topic was lonely without one.
Yeah, I did a double-take on the date...and suspect that it is, in fact, incorrect. But for one of the lists I admin (er, used to admin) here's the bottom of the chronological list of archives by year:
There are two posts in that September 1980 folder, and a few in July 1986. But one of the posts in 1986 references an HTML page at the now-closed Bethany University...and I can pretty much guarantee there wasn't an HTML page at Bethany in 1986. Because we didn't see the first one until August 1991.
I was on USENET prior to that, and helped part-time admin a university's NSFNET exchange as well as their dial-up BBS system, but I somehow doubt RootsWeb included archives from the early USENET days. The 1996 date seems reasonable, though.
At least I didn't include that 1980 date in the original question.
The RootsWeb listservs have been more active over this than they have been since the Ancestry message board cross-posting was turned off. Lists that haven't seen a message for three years have lit up. Time will tell if, in fact, the archived messages stay up for a while after the mailing lists themselves shut down.