That's pretty much it. Internet archive, google books, and usenet. I mean, there's also just websites in general - I spend many, many hours on the goog, pulling up ancient forum threads and random tilde-website reviews with minimal information. Sometimes a page that looks completely useless turns out to have one tiny keyword you haven't seen anywhere else, and it busts the whole mystery wide open. You take that back to google, and suddenly you have a dozen new leads that explain everything. Sometimes I go to google image search and scroll until I actually reach the end of the results, and then I find one thing that turns out to be a lead. It's rough.
EDIT - I forgot to mention this VERY IMPORTANT element: "before:2015"
that's the date when the collapse accelerated. you cannot search for anything whatsoever on the internet without this filter because you will simply be deluged with keyword spam. this doesn't fix it completely, but it makes a huge difference. You can use it on mobile too, and it works on google images and books, where it's even more essential.
If you're trying to look up info about a 1980s laptop, the only way you're going to find a magazine review is by going to google books and searching "toshiba t1100 before:1990." Otherwise you are just wasting your time. It also works with "after:" which filters out the accidental hits on mis-OCRed 19th-century almanacs and shit. 99% of my research is via phrases like "dell pentium after:1991 before:1997"
while more modern results may give you the key to unlocking it, for other people trying to figure this out, here's something that works well for me and is INCREDIBLY depressing:
filter out most of the flotsam of the modern web, by using the "tools" menu in desktop mode google, to have the date range end at 1/1/2014. On mobile you can't do date ranges, only "show me search results from the last day/week/month/year", but desktop mode lets you use it.
(or, 2019, which is when GPT2 was released and a lot of the terrible ML model generated articles started)
(edit: OP replied, you can use 'before:2014' instead)

