The tail-recursion modulo cons (TRMC) PR is #181, and I don’t think it fits your description. It has indeed been around for years, but the main reason is that the author has decided to work on other things than making necessary updates (which is perfectly ok, given that this is a contribution on his free time), and also that no one has stepped up to review it so far. Given that most of the delay comes from waiting on the people doing the work (the original author, or someone else willing to help; and a reviewer), I don’t see what could be done differently in our processes to make resolution faster – in particular, pressuring the authors to work more on their free time doesn’t really sound sustainable to me.
Of course we could say “if a PR looks good-in-theory but hasn’t seen much progress in a year, we close it”, and this would have been closed in a timely way, but I fail to see the benefit of that.