As Fritz Elfert points out in https://groups.google.com/d/msg/jenkinsci-dev/t_qgJ4PEOz8/9y1_ldtvDgAJ the current links to GitHub repositories are based solely on convention based on artifactId.
While that's the case for most repositories, for some, this convention doesn't apply, mostly due to unfortunately chosen artifactIds that would result in very weird repo names.
It would probably be good if the update site provided an optional element with the browsable SCM URL.
Plan of attack:
1. (Michael) Remove GitHub links for now ('changes' URLs are gone for good since we can't know SCM tags for sure, but the basic link, currently labeled 'GitHub', will be back)
2. (Daniel) Discuss with the community the reuse of the 'scm' entry in update-center.json or whether we need a new entry
3. (Daniel) Extend update-center.json with the browsable SCM URL (likely taken from /project/scm/url in the POM, or tried to extract from the developerConnection for more compatibility)
4. (Michael) Adapt the plugin site generator to add support for this element
5. (Michael) Adapt the plugin site to use this element to build links, and only link anywhere when it's set.