Hide products that have no stock, but still allow backorders.

hide_no_stock_products.php

<?php
/*
* Use case: Product has 'allow backorders' enabled to allow purchase
* of products with stock fewer than 1, but they want them to be purchasable
* if direct linked to or added to the cart in other ways.
*/
function pp_always_hide_zero_stock( $is_visible, $id ) {
    $product = wc_get_prodcut( $id );
 
    if ( $product->stock < 1 ) {
        $is_visible = false;
    }
 
    return $is_visible;
}
add_filter( 'woocommerce_product_is_visible', 'pp_always_hide_zero_stock', 10, 2 );