This morning, I tried to get a coop student up and running on an older version of the Google App Engine for Eclipse plugin.
The following website and all related links appear to have been wiped off the face of the earth:
https://developers.google.com/eclipse/
Is this just down at the moment? Is it possible to get older versions of the plugin?
That official documentation is most likely gone for good, the plugin was deprecated in favour of the Google Cloud Tools for Eclipse. From Migrating from the Google Plugin for Eclipse:
The Google Plugin for Eclipse is deprecated and will not be supported beyond Eclipse 4.6 (Neon). It will be removed in early 2018.
This document describes how to migrate a project that uses the Google Plugin for Eclipse to the supported tooling.
You can check the snapshots of the docs on the Wayback Machine, and maybe still find the matching code repositories, if they haven't been removed as well.
But it's probably a good idea to switch to the supported tools sooner than later, especially since they're just getting started.