Rails 3.1 has zero Issues
Which isn't a valid metric. There are no issues in the tracker with the "rails-3-1" label, but there are a number of issues in the components of rails (ActiveRecord et.al)
The full list of all Rails related issues: https://github.com/rails/rails/issues?sort=created&direc...
As you can see, there are quite a few for the 3.1 code base.
Because you need to look under the milestones section. Still 6 left: