DefaultAccessCheck.php

<?php

namespace Drupal\Core\Access;

use Drupal\Core\Routing\Access\AccessInterface as RoutingAccessInterface;
use Symfony\Component\Routing\Route;

/**
 * Allows access to routes to be controlled by an '_access' boolean parameter.
 */
class DefaultAccessCheck implements RoutingAccessInterface {
    
    /**
     * Checks access to the route based on the _access parameter.
     *
     * @param \Symfony\Component\Routing\Route $route
     *   The route to check against.
     *
     * @return \Drupal\Core\Access\AccessResultInterface
     *   The access result.
     */
    public function access(Route $route) {
        if ($route->getRequirement('_access') === 'TRUE') {
            return AccessResult::allowed();
        }
        elseif ($route->getRequirement('_access') === 'FALSE') {
            return AccessResult::forbidden();
        }
        else {
            return AccessResult::neutral();
        }
    }

}