This idea appeared in a twitter thread: https://twitter.com/Gankro/status/1144041406404661248. I did a quick search here and could not found anything relevant to the matter (please forgive me if this has already been discussed).
Anyway I think it would be nice to have a bot in github to close old issues in the rust-lang repos that are no longer relevant in two steps: First a warning, and then closing it if no one answered to the warning. I don’t know if this could be done (or it is already done and I am missing it) by any of the existing bots.
If you consider this is a good idea I’d be more than happy to work on it