On https://drill.apache.org/docs/rest-api-introduction/ there is a link with anchor text "Web UI" that links to a 404 page at https://drill.apache.org/docs/starting-the-web-console/
If you search google for:
You can find other links to the same page. I have recently run into another page that is linked to from a couple of places that is now a broken link.
I wonder if it would make sense to put a placeholder or redirect at that URL; if an HTTP redirect is possible that would be best, but otherwise a page that says "This page has moved to: [URL]"
If someone has setup Google Search Console I believe it has a broken links report you could use to identify other links that have been broken.