Examples:

• non-existent branch https://github.com/sindresorhus/refined-github/tree/404-branch
• existent file https://github.com/sindresorhus/refined-github/blob/shorten-url-labels/test/404.js
• deleted file https://github.com/sindresorhus/refined-github/blob/master/extension/content.js

Possible content:

• link to repo root
• link to closest existing directory
• link to last commit that had the file (example) (possible? edit: yes)

Perhaps this could be a separate extension though.