In https://github.com/WordPress/WordPress/commit/f4fc857b9c71f150a9abfa29404e9a07437ae32d#diff-6a81e4b18bb9bfe1f02588ddc35d801bR1711 Nacin made it possible for WordPress taxonomies to be filtered down in a wp.getTaxonomies request. Unless a 'public' bool value is supplied, the response will only contain public taxonomies.