Results sorting: relevance, newest, oldest

Example custom plugin based on the new plugin from the MR to order by newest to oldest by default and optionally use the "orderby" query parameter:

<?php

namespace Drupal\custom_vertex_module\Plugin\SearchOrder;

use Drupal\Core\Entity\EntityTypeManager;
use Drupal\vertex_ai_search\Plugin\VertexSearchOrderPluginBase;
use Symfony\Component\DependencyInjection\ContainerInterface;
use Symfony\Component\HttpFoundation\RequestStack;

/**
 * Custom Order.
 *
 * @VertexSearchOrderPlugin(
 *   id = "vertex_search_order_custom",
 *   title = @Translation("Custom Order"),
 * )
 */
class CustomOrder extends VertexSearchOrderPluginBase {

  /**
   * The request stack service.
   *
   * @var \Symfony\Component\HttpFoundation\RequestStack
   */
  protected RequestStack $requestStack;

  /**
   * The entity type manager service.
   *
   * @var \Drupal\Core\Entity\EntityTypeManager
   */
  protected EntityTypeManager $entityTypeManager;

  /**
   * {@inheritdoc}
   */
  public function __construct(
    array $configuration,
    $plugin_id,
    $plugin_definition,
    RequestStack $request_stack,
    EntityTypeManager $entity_type_manager,
  ) {

    parent::__construct($configuration, $plugin_id, $plugin_definition);
    $this->requestStack = $request_stack;
    $this->entityTypeManager = $entity_type_manager;
  }

  /**
   * {@inheritdoc}
   */
  public static function create(ContainerInterface $container, array $configuration, $plugin_id, $plugin_definition) {

    return new static(
      $configuration,
      $plugin_id,
      $plugin_definition,
      $container->get('request_stack'),
      $container->get('entity_type.manager'),
    );

  }

  /**
   * {@inheritdoc}
   */
  public function getSearchOrder() {
    $current_request = $this->requestStack->getCurrentRequest();

    // Default order by date published descending.
    $order_string = 'datePublished desc';

    // Get order from query string.
    $orderby = $current_request->query->get('orderby');

    // Create an order string from query parameters.
    if (!empty($orderby)) {
      $order_string = $orderby . ' desc';
    }

    return $order_string;
  }

}

This could be enhanced by an additional query parameter for whether the sort is descending or ascending.

Merged in MR. Marking as Fixed. Will be in release 1.9.0