Hook to run afer config import
The config system has switched already to events and the documentation for events describes in "My First Drupal 8 Event Subscriber" how to subscribe to a config event:

https://www.drupal.org/docs/8/creating-custom-modules/subscribe-to-and-dispatch-events

See also this example from the Color module:

/core/modules/color/src/EventSubscriber/ColorConfigCacheInvalidator.php

&lt;?php

namespace Drupal\color\EventSubscriber;

use Drupal\Core\Cache\CacheTagsInvalidatorInterface;
use Drupal\Core\Config\ConfigCrudEvent;
use Drupal\Core\Config\ConfigEvents;
use Symfony\Component\EventDispatcher\EventSubscriberInterface;

/**
 * A subscriber invalidating cache tags when color config objects are saved.
 */
class ColorConfigCacheInvalidator implements EventSubscriberInterface {

  /**
   * The cache tags invalidator.
   *
   * @var \Drupal\Core\Cache\CacheTagsInvalidatorInterface
   */
  protected $cacheTagsInvalidator;

  /**
   * Constructs a ColorConfigCacheInvalidator object.
   *
   * @param \Drupal\Core\Cache\CacheTagsInvalidatorInterface $cache_tags_invalidator
   *   The cache tags invalidator.
   */
  public function __construct(CacheTagsInvalidatorInterface $cache_tags_invalidator) {
    $this-&gt;cacheTagsInvalidator = $cache_tags_invalidator;
  }

  /**
   * Invalidate cache tags when a color theme config object changes.
   *
   * @param \Drupal\Core\Config\ConfigCrudEvent $event
   *   The Event to process.
   */
  public function onChange(ConfigCrudEvent $event) {
    // Changing a theme's color settings causes the theme's asset library
    // containing the color CSS file to be altered to use a different file.
    if (strpos($event-&gt;getConfig()-&gt;getName(), 'color.theme.') === 0) {
      $this-&gt;cacheTagsInvalidator-&gt;invalidateTags(['library_info']);
    }
  }

  /**
   * {@inheritdoc}
   */
  public static function getSubscribedEvents() {
    $events[ConfigEvents::SAVE][] = ['onChange'];
    $events[ConfigEvents::DELETE][] = ['onChange'];

    return $events;
  }

}