GitHub now lets people distinguish when closing an issue that no work was done, that it's a duplicate, etc. This distinction can be handy at times, and it helps at-a-glance readability since "completed" and "not planned" closed issues have different-looking icons.

This is being used to an extent by humans doing issue triage (e.g., visible at https://github.com/golang/go/issues?q=is%3Aissue+is%3Aclosed). When gopherbot closes issues that aren't actionable and time out waiting for required information, it too can use "not planned" as the reason.