Deployment of new website has broken old references
Search engines are still caching old URLs that no longer point to documentation, following the changeover from the old website. It's likely there are old external references to those URLs in places that can't/won't be updated also (such as mailing lists, forum threads, etc.)
I believe there's an opportunity to improve discoverability by re-adding these URLs as redirects to the appropriate article (either one that exists internally, or point to
developer-old. in cases where documentation hasn't been ported). As we're publishing on GitLab Pages, a module such as sphinx-redirects might serve useful.