https://github.com/jekyll/jekyll-redirect-from

A file /3.4/new.md with frontmatter redirect_from: /3.4/old.html will generate a page 3.4/old.html which redirects to /3.4/new.html. Because /stable is a symlink to /3.4, the redirect page will also be used for /stable/old.html, but forward you to /3.4/new.html instead of /stable/new.html - which is bad.

How to support version aliases?

• Fork plugin and favor the alias over the version number?
  It would then put you from /3.4/old.html onto /stable/new.html which is also not ideal.
• Materialized alias versions (copy /3.4 to /stable and fully build both)?
  Would require a customized plugin to substitute the redirect path...