Register taxonomy

gistfile1.txt

// Add Level taxonomy to pages.
function fsecom_register_taxonomy_level() {
	$labels = array(
		'name'          => _x( 'Level', 'taxonomy general name' ),
		'singular_name' => _x( 'Level', 'taxonomy singular name' ),
		'search_items'  => __( 'Search Levels' ),
		'all_items'     => __( 'All Levels' ),
		'edit_item'     => __( 'Edit Level' ),
		'update_item'   => __( 'Update Level' ),
		'add_new_item'  => __( 'Add New Level' ),
		'new_item_name' => __( 'New Level Name' ),
		'menu_name'     => __( 'Levels' ),
	);
	$args   = array(
		'hierarchical'      => false,
		'labels'            => $labels,
		'public'            => true,
		'show_ui'           => true,
		'show_admin_column' => true,
		'query_var'         => true,
		'show_in_rest'      => true,
	);
	register_taxonomy( 'level', [ 'page' ], $args );
}
add_action( 'init', 'fsecom_register_taxonomy_level' );