WooCommerce weight boost for products in stock.

rlv_woo_stocklevel_boost.php

<?php

// Add this to theme functions.php.

add_filter( 'relevanssi_match', 'rlv_woo_stocklevel' );
function rlv_woo_stocklevel( $match ) {
  $in_stock = true;
  if ( has_term( 'outofstock', 'product_visibility', $match->doc ) ) {
    // Product is not in stock.
    $in_stock = false;
  }
  if ( $in_stock ) {
    // If product is in stock, multiply the weight by 10.
    $match->weight *= 10;
  }
  return $match;
}