With redirect.rd_fragment now implemented, it should be possible to list redirects where the target contains a broken section anchor. For example, if the redirect page "Baz" contains "#REDIRECT [[Foo#Bar]]" and the "Bar" section no longer exists on the page "Foo", "Baz" would be listed somewhere in the user interface.
This could be implemented in a separate special page or in Special:BrokenRedirects.
Severity: enhancement
There's still no guarantee that the target anchor of the redirect is pointing to a section as opposed to a hand made <div id="blah"> thing. However, ignoring that we also don't store what sections actually exist on a page (as far as I'm aware) in the db. So there's still a fair bit more to do before that can happen.
Another idea is to use the Linter extension and its framework to add a tracking category to redirect pages that have invalid/broken section anchors. This would require getting the Linter extension to work with a non-Parsoid "external service" and would require solving the challenges @Bawolff mentions about manually added IDs.
After the redirect target page is touched and we have the rendered HTML, we could look for all IDs, whether manually added or via a section link, then look up section redirects, and then record the missing ones as lint errors (or somewhere). And then we'd have to do the same whenever the redirect page itself is edited and contains a fragment.