You can find official Shimano spec sheets on their website. They reach back to the year 2004. Do older spec sheets from the 80s and 90s exist?
Yes they do.
They way Shimano did it for a very long time (I don't have the date range) was that each model of component released had two different documents released for it, which were Shimano's primary means of communicating technical specs to the world. These were the "SI" (Service Instructions) and "EV" (Exploded View) documents. The SIs contained throwaway legalese, setup instructions, and compatibility type information, for example total capacities and max large cog numbers on a rear derailleur, or recommended spindle length for a crank. Sometimes the compatibility type information you need is actually hiding on the EV, especially when what you're trying to figure out is a small part number, i.e. for cross-compatibility parts ordering type purposes.
The EVs still exist in the same form as ever, but at some point (again I don't recall the year), they stopped doing SI documents and instead now do the UMs (User Manuals) and DMs (Dealer Manuals) found on si.shimano.com for the procedural and cautionary/legalese type content, and their spec tables for compatibility information.
As far as I know, the 2004 date you mention was the beginning of Shimano releasing spec data in table form. SIs were still around then, so in this period both types of document were released and had some of the same information. For parts not covered in those archives, the main source for that kind of data is the SI documents. There are some entities out there who have taken it upon themselves to archive them for the public, and a lot of those dating pretty far back in time can be found by searching the model number plus "si". Shimano has also added some older SI documents to their own site.
Prior to the Dealer Manual PDFs, their current form of communicating core procedural information, there used to be yearly paper "STI Guides" that dealers got. It was a maddening and backwards system because there was some technical material in them that wasn't available anywhere else, but getting old ones was difficult, and so was finding what you needed even if you did have a huge stack of them. They were also the source of compatibility charts akin to what's in the Compatibility section of si.shimano.com now. STI Guides persisted well into the internet age, but I'm not sure when the last one came out.
In some cases, but not in the same format and note that those links are not complete in that they omit information provided elsewhere
here are some links:
various Shimano catalogs back to 1973 which vary between collections of exploded views or spec lists, line-up charts, etc: http://www.disraeligears.co.uk/Site/Documents_-_Shimano.html
1974 Shimano Bicycle Parts with collections of exploded views https://www.flickr.com/photos/stronglight/sets/72157625399206479/
various catalogues https://vintagecannondale.com/info/shimano/
various early-2000s compatibility charts
various catalogues here http://www.retrobike.co.uk/gallery2/v/Manufacturer+Archive/Shimano+Archive/Catalogues/
various other places e.g. this 1993 catalogue scan http://www.tearsforgears.com/2009/11/1993-shimano-ultegra-600.html http://www.tearsforgears.com/2009/10/1993-shimano-catalog.html
1994 catalogue here http://cyclespeugeot.web.fc2.com/reminiscence/shimano94.htm
web.archive.org e.g. this 2002 Dura-Ace 7700 which is different from information now available online http://web.archive.org/web/20020225203704/http://bike.shimano.com/road/dura-ace/index.asp or this 41-page 2003 disc brake catalog which provides technical information and installation details or this from the EU Shimano web catalog on the differences between the 7700 RD http://web.archive.org/web/20040119025927fw_/http://www.shimano-eu.com/cycling/catalogue/content_prodx.phtml?pid=10102&psid=36 and the 5500 RD http://web.archive.org/web/20040131113619fw_/http://www.shimano-eu.com/cycling/catalogue/content_prodx.phtml?pid=10124&psid=38 (which provides far more detail than in the archived spec lists available online - e.g., on pivot seals or their absence) http://web.archive.org/web/20030509114347/http://bike.shimano.com/services/images/english/Shimano_Disc_Brake_Systems.pdf (try Bike.shimano.com, shimano.com/cycling cycle.shimano.co.jp)
the list of Shimano part numbers from 1980 to 2006 from Allgemeiner Deutscher Fahrrad Club http://www.fa-technik.adfc.de/Hersteller/Shimano/Gruppen2000.html http://www.fa-technik.adfc.de/Hersteller/Shimano/Gruppen.html http://www.fa-technik.adfc.de/Hersteller/Shimano/Gruppen80.html This is probably better than any other source for finding groupset data.
using the preceding enter the part number into https://si.shimano.com/ where most Shimano parts back to about 1980 are now detailed, although not all printed documents have been scanned and the absence of a part is not proof it never existed.
sites such as 'weight weenies', where for example you can find the weight of obsolete Shimano parts that pre-date their 2004 catalogs online https://weightweenies.starbike.com/listings.php
Usenet archives; around 2000 the best source of information would have been Usenet discussions. In particular https://groups.google.com/g/rec.bicycles.tech/
Try https://si.shimano.com/ Search for the model or series number you want. The manuals go back a way, but not forever. Some series/parts more than others. If you go to 105, for instance, there are tabs for R7000, 5800, 5700, 5600, 5500 and ARCHIVE. It's easy enough to navigate.
Having had a browse around, there are a significant number of older documents added from last time I had a look, early 90s or even late 80s,