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Thanks Doug. Really an eye-opener.

Hey There, I would recommend writing into us at help@moz.com with your campaign information so that we can look into the issue more directly and forward the information over to our engineers if necessary. We look forward to hearing from you there soon. Chiaryn

Hi Martijn,  This sitemap doesn't contain images at all I can't put the URL here in plain text, please see attached image. sitemap-issue-moz.png

Hard to say what is meant by that video. Often, Google is purposely vague. If the content is truly no longer relevant, I would 301 it to more relevant URLs on a page by page basis.  This will remove low performing pages from Google's index, and potentially improve your rankings. On the other hand, if the content still has value but doesn't need to be front and center, a clearly organized archive based on date or some other organizational method should work fine.