Obviously Links have their own Category, but this Category is not listed in Visibility Graphics \ Model Categories. The Link Category can be hidden by accident, when hiding other Categories, so that all links are hidden, but this is not displayed in the Visibility Graphics dialog, so the only way of turning the Link category back on is to use the Reveal Hidden Elements lightbulb tool, and selecting a Link and clicking Unhide Category.
As there is a Category for Links this should be listed in the Visibility Graphics \ Model Categories.
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