Before that it was 20 years of BBS content that is sadly mostly lost.
Of course, there's also a huge amount of information about companies, products, news, etc. that was largely never in electronic form and--where it didn't just get tossed in the trash in the wake of some corporate buyout--is in the stacks of some library someplace.
And unfortunately Google has starting blocking some groups because of too much spam (that to some extent probably was enabled by Google itself in the first place), and unfortunately that means that the archives of those groups (pre-dating the current spam deluge) are inaccessible, too.