No, not reopen. Simply because of GitHub. If you reopen an issue, you’ll notify only previous commenters, but if you open a new issue, you reach a greater audience. And for nixpkgs, that audience changes each year. You will notify previous commenters by mentioning that old issue in a new one.
Also, you can’t reopen PRs.
One more argument for closing staled issues. PR Improvements for declarative nixos containers by chrisfarms · Pull Request #3021 · NixOS/nixpkgs · GitHub. It should have been marked “stale” (or “volunteer-wanted”) and closed in 2015-2016. Other PRs reimplemented it’s parts (in some cases, new authors didn’t know about this PR existence!), so it was of little use to keep it open.
And also, I’ve started a list of nixpkgs issues and PRs, which are quite interesting, but decision is hard to impossible to do: User:Danbst/Ideas in Limbo - NixOS Wiki . Feel free to extend this list!