Add "Sort by discount" to sorting options. Defaults to biggest to smallest discount.

sort-by-discount.php

<?php

/**
 * Add "Sort by discount" to sorting options. Defaults to biggest to smallest discount.
 */
add_filter( 'woocommerce_get_catalog_ordering_args', 'mycode_woocommerce_add_salediscount_to_catalog_ordering_args' );
function mycode_woocommerce_add_salediscount_to_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 ( 'discount' == $orderby_value ) {
		$args['orderby'] 	= 'meta_value_num';
		$args['order'] 		= 'DESC';
		$args['meta_key'] 	= '_dfrps_salediscount';
	}
	return $args;
}

add_filter( 'woocommerce_default_catalog_orderby_options', 'mycode_woocommerce_add_salediscount_to_catalog_orderby' );
add_filter( 'woocommerce_catalog_orderby', 'mycode_woocommerce_add_salediscount_to_catalog_orderby' );
function mycode_woocommerce_add_salediscount_to_catalog_orderby( $sortby ) {
	$sortby['discount'] 	= 'Sort by discount';
	return $sortby;
}