While I am unable to locate it in the current guides up in the forums, there was a point when ruins were considered valid criteria (5 stars), especially if they had some historic value and/or were part of a hiking trail or park. Recently I nominated a set of historic ruins (1700s-1800s, this site was one of the early areas of industry in our state), with mention of the significance behind these ruins. The nomination was rejected.
Did I just not take a clear enough photo, or has the criteria for ruins been changed? The rejection reason was "Natural feature". The candidate is located in what is now a nature preserve, but the stonework tailrace was supposed to be the focus of both the candidate and supporting photo.
Information on this site is located at: https://www.ehlt.org/hammond-mill-preserve