Dublin Core

 Dublin Core  (AKA Dublin Core Schema or just DC) is an old set of vocabulary that was used in meta tags for terms like date, subject, author of a page that has few if any IndieWeb publishing examples beyond Known support, and has no known non-trivial consuming code or services for web pages.

Dublin Core metadata uses a "DC:" prefix inside the meta tag name attribute.

IndieWeb Examples
Other than Known websites publishing Dublin Core meta tags, there are known no examples of #indieWeb sites prominently publishing or any that use or consume the vocabulary for any actual purpose.

Other Examples

 * Hypothesis has a tiny bit of obscure Dublin Core document identity vocabulary consuming support when parsing documents per https://web.hypothes.is/help/how-hypothesis-interacts-with-document-metadata/ "When Hypothesis sees the combination of dc.identifier and dc.relation.ispartof it joins the values of the two tags to create a URL-independent identifier."

Too ambiguous
Dublin Core does not unambiguously identify important document aspects. "Use Dublin Core tags (e.g., DC.title) as a last resort - they work poorly for journal papers because Dublin Core doesn't have unambiguous fields for journal title, volume, issue, and page numbers. Google Scholar Inclusion Guidelines"

Lack of consuming code for HTML

 * Citation for lack of HTML consuming of DC (note authority) https://twitter.com/paulwalk/status/1093110979406237697
 * "(disclaimer: I manage DCMI): Good question :-) There is evidence of DCMI Terms being used in Linked Data in other contexts, but not aware of research into consumption of DC in context of being embedded in webpages. [1/2]" @paulwalk February 6, 2019

DRY violation

 * Due to the paucity of consumers including Dublin Core meta tags typically duplicates visible information and thus violates the principles of DRY.