Points of note:
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An unlisted topic will not show up in a search using the Discourse search, even as a category moderator they do not show up in a search which makes them hard to find at times.
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A topic that is listed then converted to unlisted may have been indexed by a search engine such as Google before becoming unlisted. As such while these topics are hard to find with a Discourse search they are easier to find with a Google search even though one of the points of unlisting them is to keep them from being found with a search engine.