PRs in that repo are a much bigger deal than issues: issues are a place to talk in a general sense and see if others are interested in the feature, but converting that into an actual, actionable RFC is a lot of work. I'm not aware of any actual RFC for non-lexical lifetimes, because everyone who would do such a thing knew we had to wait for MIR to shake out before they were realistically possible, and so writing an RFC before then would mostly be a waste of time.