Websites are easy to create and add to compared with maintaining them. If you use something like Drupal or WordPress there is the labour of keeping up to date with releases, especially if you use third-party plugins. Even if you use just PHP and MySQL you may well have to keep up to date with releases of those.
One aspect of keeping a website healthy is maintaining links to external websites. Even in the first paragraph we have four links which need checking regularly. So on a regular basis I run a script which loops over all the links on the generated website and checks they seem like good links.
This script is still evolving together with what to do about problem links. For some links the website has just vanished. For other links a 301 or 308 means one can just change the link to the redirection location.