I just ran into this service that has plugins for Wordpress and Drupal:
Amber is an open source tool for websites to provide their visitors persistent routes to information. It automatically preserves a snapshot of every page linked to on a website, giving visitors a fallback option if links become inaccessible.
If one of the pages linked to on this website were to ever go down, Amber can provide visitors with access to an alternate version. This safeguards the promise of the URL: that information placed online can remain there, even amidst network or endpoint disruptions.
Amber is an open source project led by the Berkman Klein Center for Internet & Society at Harvard University.
The overall problem space is described in The End of Infinite Data Storage Can Set You Free | WIRED