One reason is if you keep all your old versions of product docs up, Google will randomly send people to the old version instead of the current one, and then customers will get confused by the outdated info
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
And you're probably out of support and aren't a current paying customer so you're a lower priority than current customers that want current information. (Old information is often available on product support sites but it probably won't come up with a random search.)
When I worked at $MAJOR_VENDOR, we had an internal-only archive of product installation and documentation ISOs for old versions, discontinued products, etc. It didn't go back forever (although apparently some even older stuff was archived offline and could be retrieved on request), but it did contain product versions that had been out-of-support for over a decade. A lot of older material which had been removed from the public website was contained in it.
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.
And for product documentation for version n-2.x, it's easy enough to see the use case for why a customer might need it. (e.g. they have some other product that's version locked to it) But there is a ton of collateral, videos, etc. that companies produce that get stale, musty, or just plain wrong and if you leave them all out there it's just a mess that's now up to the customer to decide what's right, what's mostly right, and what's plain wrong.
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.
> But there is a ton of collateral, videos, etc. that companies produce that get stale, musty, or just plain wrong
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.
Or your 2011 presentation about cloud computing didn't mention containers. I'm not sure it's embarrassing any more than a zillion other forward looking crystal ball glimpses is embarrassing. But there's no real reason to keep on your site unless your intent is to provide a view into technology's twisty path or the various false starts every company makes with its products.
Companies are not, in general, in the business of being the archive of record. They're, among other things, in the business of providing their current customers with the information that's most applicable and correct for their current needs and not something relevant to a completely different version of software from 10 years ago.
My inner historian is screaming.