Indicate which pages are taxonomy archives

taxonomy_archive_indication.php

add_filter( 'display_post_states', 'cnmd_add_archive_page_indicators', 10, 2);
/**
 * cnmd_add_archive_page_indicators adds post state labels for custom and builtin taxonomy
 *  archive pages
 *
 * @param ARRAY $post_states
 * @param POST OBJ $post
 *
 * @return array
 */
function cnmd_add_archive_page_indicators($post_states, $post) {

	// Get the taxonomy slug, since this is what will be used to find the archive page
	$slug = $post->post_name;
	if ( ! $slug ) {
		return $post_states;
	}

	// Get the name of each custom public registered taxonomy
	$args = array(
		'public'   => true,
		'_builtin' => false,
	);
	$custom_taxonomies = get_taxonomies( $args, 'objects' );

	$new_post_state = false;

	if ( ! empty( $custom_taxonomies ) ) {
		$new_post_state = cnmd_get_archive_page_post_state( $custom_taxonomies, $slug );
		if ( $new_post_state ) {
			$post_states[] = $new_post_state;
		}
	}

	if ( ! $new_post_state ) {
		// Get the name of each built in registered taxonomy
		$args = array(
			'public'   => true,
			'_builtin' => true,
		);
		$builtin_taxonomies = get_taxonomies( $args, 'objects' );

		if ( ! empty( $builtin_taxonomies ) ) {
			$new_post_state = cnmd_get_archive_page_post_state( $builtin_taxonomies, $slug );
			if ( $new_post_state ) {
				$post_states[] = $new_post_state;
			}
		}
	}
	return $post_states;
}

/**
 * cnmd_get_archive_page_post_state returns a string if $slug matches any of the supplied $taxonomies slugs,
 * which indicates that the page is the archive page for that taxonomy.
 *
 * @param ARRAY $taxonomies Array of taxonomy info objects
 * @param STRING $slug Slug of current page
 *
 * @return string
 */
function cnmd_get_archive_page_post_state( $taxonomies, $slug ) {
	if ( empty ( $taxonomies )) {return;}
	if ( '' === $slug ) { return; }
	foreach ( $taxonomies as $taxonomy ) {
		if ( empty ( $taxonomy->rewrite ) || ! array_key_exists('slug', $taxonomy->rewrite) || '' === $taxonomy->rewrite[slug]) {continue;}
		if ( $slug === $taxonomy->rewrite[ slug ] ) {
			return  "Archive page for " . $taxonomy->labels->name ;
		}
	}
	return;
}