I don't think it's a good idea to make this dependent on github. The README or VERSION file could contain a note about the project status/quality: "version 1.2 -- abandoned". Simply creating an empty file named ABANDONED would work too for people who don't use github.
Indeed. What would be a nice addition to github is a way to mark another repository as the canonical one. I've been lucky enough to have a couple of my old projects taken over by new maintainers, and it takes a while for people to realize it.
What if the user created a file named ABANDONED.rdoc and then Github displayed it before the README.rdoc file. It could even have a different color or something to make it stand out. That way Github doesn't have to add a new field to a form and the idea still works even off-line.
