How to deal with obsolete answers?
Given an old question that has old answers that were correct when answered. Now consider that something has changed that makes one or more answers no longer correct.
For example, this answer was correct for the then-current Nokogiri version, but is no longer correct. It's flat out wrong (for the current version of the library).
As another example, the accepted answer to this question was once correct (no such functionality existed) but is now wrong (the functionality now exists).
Is it acceptable to downvote these answers as they are wrong? Upside: this helps highlight more correct answers. Downside: this punishes (lightly) a helpful user who gave a good answer.
What if a downvote is always accompanied by a comment notifying the user that they should delete or edit their answer, and then a followup removal of the downvote iff an edit occurs?