The electronics firm I mentioned a couple of days ago have now closed.
They've done this by, amongst other things, closing down their wirelessthings.net website.
The one that had the technical details of the stuff they were selling: important information necessary to use it.
They previously had two other sites on ciseco.co.uk and openmicros.org – the former had the shop, the latter had things like the documentation, source code and discussion forum.
Archive.org has a good selection of pages from all three sites, except that it won't show them. Both the older domains redirect to the newer one. Partly because it was built using a completely different content management system (CMS), it has a robots.txt file that says to any vaguely responsible search engine 'don't look here' at exactly the place the CMS on the older sites used to display everything. So archive.org, being vaguely responsible, looks at the page you want (say openmicros.org/index.php/importantstuff), checks openmicros.org/robots.txt, gets redirected to wirelessthings.net/robots.txt, sees that it's not to show anything from /index.php/ and says 'won't'. Google has copies of many pages too and won't show it for the same reason.
I emailed the firm last week saying this was a problem and needed fixing, but it's still happening.
I have copies of the stuff I need, and one critical library is on github, but the irresponsibility of unnecessarily taking your documentation off the web is appalling. I'm glad they've gone.