It always sounds like a great idea to update the readme file, but I have to admit that I almost never read them. I think that an admin notice about the last version with support is a much better option.

Another issue we have (though I have no idea how widespread it is) is 2 year old plugins that don't show up in search anymore, but work for their intended use just fine. WP-DB-Backup is a great example of that. Works just fine, but to install it you have to go to the .org repo and download it.

I have no solution since the 2 year mark seems like a great way to tell if a plugin is still around but it's not a blanket way to tell if a plugin is dead.