Forcing all menu links to use absolute links in Drupal 8/9

MYTHEME.theme

<?php

/**
 * Implements hook_preprocess_menu().
 *
 */
function MYTHEME_preprocess_menu(&$variables) {
  // Limit to specific menus for performance.
  $menus = ['main', 'footer'];
  if (isset($variables['menu_name']) && in_array($variables['menu_name'], $menus)) {
    MYTHEME_set_menu_items_to_absolute($variables['items']);
  }
}

/**
 * Recursively make menu items link with absolute.
 *
 */
function MYTHEME_set_menu_items_to_absolute(&$items) {
  foreach ($items as $item) {
    $url = $item['url'];
    if (!$url->isExternal()) {
      $item['url'] = $url->setOption('absolute', TRUE);
    }
    // Recursively loop of sub-menu items.
    if (isset($item['below'])) {
      MYTHEME_set_menu_items_to_absolute($item['below']);
    }
  }
}