Why is there a module for version 9 of core?
Well the "Bad Judgment" module you linked to claims:


  This module provides an explicit dependency for modules whose usage requires bad judgement. Joke modules often require "bad judgement".


So I wouldn't take that version number too seriously.

From the core download page you linked to:


  Placeholder for the Drupal 9.x release. Not intended for actual use by anyone except the issue queue version selector.


It's just a placeholder for the next version (forward-thinking and all that). It doesn't mean there's anything wrong with Drupal 8, and it doesn't mean Drupal 9 will be out any time soon.