LayoutTestThemeHooks.php

<?php

declare (strict_types=1);
namespace Drupal\layout_test\Hook;

use Drupal\Core\Hook\Attribute\Hook;

/**
 * Hook implementations for layout_test.
 */
class LayoutTestThemeHooks {
  
  /**
   * Implements hook_preprocess_HOOK() for layout templates.
   */
  public function templatePreprocessLayoutTest2col(&$variables) : void {
    $variables['region_attributes']['left']->addClass('class-added-by-preprocess');
  }

}