This has been requested internally on two separate occasions. Essentially, sometimes a url might no longer be accessable. Perhaps someone moved the content somewhere else, perhaps the site is closed completely etc.
Would it be feasible to notify a user when a url in one of their topics returns a 404?
I can see an issue where some sites may require an API key to read the particular link, or some other permissions restrictions, so that would need to be factored in. However in general this would be quite a useful feature in ensuring content is renewed as soon as possible.