Add register_taxonomy flag

src/wp-includes/class-wp-query.php

@@ -1173,9 +1173,9 @@ public function parse_tax_query( &$q ) {
 				continue;
 			}
 
-			if ( 'category' === $taxonomy && empty( $q[ $t->query_var ] ) ) {
-				// Unlike custom taxonomies, the category field is automatically added to every query.
-				// Thus, we need to skip it if it is empty.
+			if ( empty( $q[ $t->query_var ] ) && ! $t->root_taxonomy_archive ) {
+				// We only allow the term to be empty if the taxonomy is set up to
+				// show its root archive.
 				continue;
 			}
 

src/wp-includes/class-wp-taxonomy.php

@@ -245,6 +245,15 @@ final class WP_Taxonomy {
 	 */
 	public $default_term;
 
+	/**
+	 * Whether to show a page for the root taxonomy route, displaying all
+	 * posts that have any term from the taxonomy assigned.
+	 *
+	 * @since 6.8.0
+	 * @var bool
+	 */
+	public $root_taxonomy_archive = false;
+
 	/**
 	 * Whether terms in this taxonomy should be sorted in the order they are provided to `wp_set_object_terms()`.
 	 *
@@ -359,6 +368,7 @@ public function set_props( $object_type, $args ) {
 			'rest_namespace'        => false,
 			'rest_controller_class' => false,
 			'default_term'          => null,
+			'root_taxonomy_archive' => false,
 			'sort'                  => null,
 			'args'                  => null,
 			'_builtin'              => false,

src/wp-includes/taxonomy.php

@@ -426,6 +426,7 @@ function is_taxonomy_hierarchical( $taxonomy ) {
  * @since 5.4.0 Added the registered taxonomy object as a return value.
  * @since 5.5.0 Introduced `default_term` argument.
  * @since 5.9.0 Introduced `rest_namespace` argument.
+ * @since 6.8.0 Introduced `root_taxonomy_archive` argument.
  *
  * @global WP_Taxonomy[] $wp_taxonomies Registered taxonomies.
  *
@@ -506,6 +507,8 @@ function is_taxonomy_hierarchical( $taxonomy ) {
  *         @type string $slug         Slug for default term. Default empty.
  *         @type string $description  Description for default term. Default empty.
  *     }
+ *     @type bool          $root_taxonomy_archive Whether to show a page at the root taxonomy route, displaying all
+ *                                                posts that have any term from the taxonomy assigned. Default false.
  *     @type bool          $sort                  Whether terms in this taxonomy should be sorted in the order they are
  *                                                provided to `wp_set_object_terms()`. Default null which equates to false.
  *     @type array         $args                  Array of arguments to automatically use inside `wp_get_object_terms()`