WooCommerce: Sort by Stock Level

<?php
/**
 * Filter: Add a custom sort to WooCommerce: Quantity
 * Author: Barbi Atkinson (@akyusa01)
 * Version 1.0
 */

add_filter( 'woocommerce_get_catalog_ordering_args', 'custom_woocommerce_get_catalog_ordering_args' );

function custom_woocommerce_get_catalog_ordering_args( $args ) {
$orderby_value = isset( $_GET['orderby'] ) ? woocommerce_clean( $_GET['orderby'] ) : apply_filters( 'woocommerce_default_catalog_orderby', get_option( 'woocommerce_default_catalog_orderby' ) );

if ( 'stock_levels' == $orderby_value ) {
$args['meta_key'] = '_stock';
$args['orderby'] = 'meta_value_num';
$args['order'] = 'desc';
}

return $args;
}

add_filter( 'woocommerce_default_catalog_orderby_options', 'custom_woocommerce_catalog_orderby' );
add_filter( 'woocommerce_catalog_orderby', 'custom_woocommerce_catalog_orderby' );

function custom_woocommerce_catalog_orderby( $sortby ) {
$sortby['stock_levels'] = 'Stock Levels';
return $sortby;
}