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

if ( 'sku' == $orderby_value ) {
	$args['orderby'] = 'meta_value';
	$args['order'] = 'asc'; // lists SKUs alphabetically 0-9, a-z; change to desc for reverse alphabetical

	$args['meta_key'] = '_sku';
}
return $args;
}
add_filter( 'woocommerce_get_catalog_ordering_args', 'custom_add_sku_sorting' );

function custom_sku_sorting_orderby( $sortby ) {
$sortby['sku'] = __( 'Sort by SKU', 'textdomain' );
return $sortby;
}
add_filter( 'woocommerce_catalog_orderby', 'custom_sku_sorting_orderby' );
add_filter( 'woocommerce_default_catalog_orderby_options', 'custom_sku_sorting_orderby' );