My inner historian is screaming.
I’m sure there must be some way to fix this with META tags/etc, but it is often easier just to delete the old stuff, than change the META tags on 100s or 1000s of legacy doc pages
There was a defined process for customers to request access to it – they'd open a support ticket, tell us what they wanted, the support engineer would check the licensing system to confirm they were licensed for it, then transfer it to an externally accessible (S)FTP server for the customer to download, and provide them with the download link. There was a cron job which automatically deleted all files older than 30 days from the externally-accessible server.
So, while there may be some historical interest in how we were talking about, say, cloud computing in 2010, that's the kind of thing I keep in my personal files and generally wouldn't expect a company to keep searchable on its web site.
That 2017 product roadmap slide saying "we'll deliver X version N+1 by 2020" gets a bit embarrassing when 2023 rolls around and it is still nowhere to be seen. Maybe the real answer is "X wasn't making enough money so the new version was cancelled", but you don't want to publicly announce that (what will the press make of it?), you just hope everyone forgets you ever promised it. You can't get rid of copies of the slide deck your sales people emailed to customers, or you presented at a conference, or in the Internet Archive Wayback Machine – but at least you can nuke it off your own website. Reduces the odds of being publicly embarrassed by the whole thing.
There are infrastructure costs and manage costs.