Show Out of stock products at the end in Woocommerce

This is the same as Viktor & Bogdan's answer, but without the extra Class code.

It uses the post_clause filter to modify the product query. We JOIN the wp_postmeta table to the query and prepend an orderby _stock_status clause to the existing query. This way any other orderby clauses remain in the query as well.

add_filter('posts_clauses', 'order_by_stock_status');
function order_by_stock_status($posts_clauses) {
    global $wpdb;
    // only change query on WooCommerce loops
    if (is_woocommerce() && (is_shop() || is_product_category() || is_product_tag() || is_product_taxonomy())) {
        $posts_clauses['join'] .= " INNER JOIN $wpdb->postmeta istockstatus ON ($wpdb->posts.ID = istockstatus.post_id) ";
        $posts_clauses['orderby'] = " istockstatus.meta_value ASC, " . $posts_clauses['orderby'];
        $posts_clauses['where'] = " AND istockstatus.meta_key = '_stock_status' AND istockstatus.meta_value <> '' " . $posts_clauses['where'];
    }
    return $posts_clauses;
}

You could change istockstatus.meta_value ASC to istockstatus.meta_value DESC if you for some reason wanted the Out Of Stock items first.

Tested on WP: 4.8; WC 3.0.8

Here is a snippet for rearranging products (in stock come first):

<?php

/**
 * Order product collections by stock status, instock products first.
 */
class iWC_Orderby_Stock_Status
{

    public function __construct()
    {
        // Check if WooCommerce is active
        if (in_array('woocommerce/woocommerce.php', apply_filters('active_plugins', get_option('active_plugins')))) {
            add_filter('posts_clauses', array($this, 'order_by_stock_status'), 2000);
        }
    }

    public function order_by_stock_status($posts_clauses)
    {
        global $wpdb;
        // only change query on WooCommerce loops
        if (is_woocommerce() && (is_shop() || is_product_category() || is_product_tag())) {
            $posts_clauses['join'] .= " INNER JOIN $wpdb->postmeta istockstatus ON ($wpdb->posts.ID = istockstatus.post_id) ";
            $posts_clauses['orderby'] = " istockstatus.meta_value ASC, " . $posts_clauses['orderby'];
            $posts_clauses['where'] = " AND istockstatus.meta_key = '_stock_status' AND istockstatus.meta_value <> '' " . $posts_clauses['where'];
        }
        return $posts_clauses;
    }
}

new iWC_Orderby_Stock_Status;

?>

https://www.snip2code.com/Snippet/114858/WooCommerce-Products-Order-by-Stock-Stat

I tried all the solutions above, they worked but resulted in other issues on the site (probably due to theme conflict), Im sure they are all good in different situations / themes. The below code finally worked great for me (source mentioned)

source: https://www.businessbloomer.com/woocommerce-show-in-stock-products-first-shop/

/**
 * @snippet       Sort Products By Stock Status - WooCommerce Shop
 * @how-to        Get CustomizeWoo.com FREE
 * @author        Rodolfo Melogli
 * @compatible    WooCommerce 3.9
 * @donate $9     https://businessbloomer.com/bloomer-armada/
 */
 
add_filter( 'woocommerce_get_catalog_ordering_args', 'bbloomer_first_sort_by_stock_amount', 9999 );
 
function bbloomer_first_sort_by_stock_amount( $args ) {
   $args['orderby'] = 'meta_value';
   $args['order'] = 'ASC';
   $args['meta_key'] = '_stock_status';
   return $args;
}