Another problem is that even if sites and their content stay up they often reorganize it for various reasons - often by importing old content into some new platform - and don't care about the URLs the content is available at. Which breaks all links to it.

Some pages at least try to show you a page with suggestions what you might've been going for, but I've also seen those less and less over the years.

For my stuff I've been making sure to keep links working for over two decades now - on my personal page you can still access everything similary to /cgi-bin/script.cgi?page even though that script and the cgi-bin directory as a whole has been gone for over a decade. But I seem to be pretty alone in efforts trying to keep things at stable locations.