Non reusable access checkers can be defined directly in the routing definition

Some access checkers does not really make sense to be reused outside of a single route.
One good example is some really specific access checker of the toolbar module:

/**
 * @file
 * Contains \Drupal\toolbar\Access\SubtreeAccess.
 */

namespace Drupal\toolbar\Access;

use Drupal\Core\Access\StaticAccessCheckInterface;
use Symfony\Component\HttpFoundation\Request;
use Symfony\Component\Routing\Route;
use Drupal\Core\Session\AccountInterface;

/**
 * Defines a special access checker for the toolbar subtree route.
 */
class SubtreeAccess implements StaticAccessCheckInterface {

  /**
   * {@inheritdoc}
   */
  public function appliesTo() {
    return array('_access_toolbar_subtree');
  }

  /**
   * {@inheritdoc}
   */
  public function access(Route $route, Request $request, AccountInterface $account) {
    $hash = $request->get('hash');
    return ($account->hasPermission('access toolbar') && ($hash == _toolbar_get_subtrees_hash())) ? static::ALLOW : static::DENY;
  }

}

For these cases you can specify a controller like method on the routing definition:

toolbar.subtrees:
  path: '/toolbar/subtrees/{hash}'
  defaults:
    _controller: '\Drupal\toolbar\Routing\ToolbarController::subtreesJsonp'
  requirements:
    _custom_access:  '\Drupal\toolbar\Routing\ToolbarController::checkSubTreeAccess'

and add the method directly on the other controller:

class ToolbarController extends ControllerBase {

  /**
   * Checks access for the subtree controller.
   */
  public function checkSubTreeAccess(Request $request) {
    $hash = $request->get('hash');
    return ($this->currentUser()->hasPermission('access toolbar') && ($hash == _toolbar_get_subtrees_hash())) ? AccessInterface::ALLOW : AccessInterface::DENY;
  }

 }