more than a service (the service provider could also disappear), it should be a function of the publishing platform
There's an extension called Resurrect Pages that modifies Firefox's "Page not Found" screen, so you can pull the URL from Google's cache or the Internet Archive. https://addons.mozilla.org/en-US/firefox/addon/resurrect-pag...
* make a copy of all linked resources when each post is published
* regularly compare the linked resources to the copies to make sure the links still work as expected
* when a link dies, automatically replace it with a link to the copy