woocommerce-exclude-out-of-stock-from-related.php

// remove out of stock products from Woocommerce related products
add_filter( 'woocommerce_related_products', function ( $related_posts, $product_id, $args ) {
    $outofstock_product_ids = (array) wc_get_products( array(
          'status'       => 'publish',
          'limit'        => -1,
          'stock_status' => 'outofstock',
          'return'       => 'ids',
      ) );

    $exclude_ids = $outofstock_product_ids;

    return array_diff( $related_posts, $exclude_ids );
}, 10, 3 );