Hi,
1. I mean to turn that setting OFF. Currently items, which have no assigned terms for the filtered taxonomy are also shown. When that option is disabled, the plugin will exclude every item, which has not been assigned any term from the given taxonomy.
2. I’m afraid not, because it would require a table join operation on the full wp_posts and wp_postmeta table (when fetching the values) – and that could take seconds, delaying the page loading greatly. We are adding an “exclude” attribute for the {get_values} pseudo variable within 2 releases, that should be a possible solution.
3. You are right, it was actually removed in 4.21, and I forgot to update the documentation with the relevant information.