There was a push a few years ago (2017 IIRC) to secure academic networks against espionage. A lot of research websites hosted on public IPs owned by universities (some of them didn't even bother to register a domain name) were suddenly only reachable on the intranet. Some of them were later made public again (probably after going through an approval process) but old sites with few users or where the only users are on the university network were never updated.
Then there's general link rot. Once I wanted to check the source of some economic statistics cited in a paper, only to discover that the domain was now being used for porn.