The requirements that a feed must observe in order to be added in Drupal Planet report that Drupal Planet posts must be tagged with , but it is not required the tag is visible.
If the tag is not visible, it's not possible to verify if the feed follows the requirements.
In the case posts are written by more users, using a visible tag makes more evident that posts tagged with appears on Drupal Planet.
Fixed
Policy
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It's likely this issue predates Contrib.social: some issue and comment data are missing.