Drupal borrows from Symfony's event dispatcher and subscriber system, but previously used its own custom event dispatcher as Symfony's was not considered performant enough. Upstream changes in the Symfony event dispatcher have improved performance and so to avoid future maintenance and take advantage of other Symfony improvements, Drupal now uses Symfony's event dispatcher.
Drupal\Component\EventDispatcher\ContainerAwareEventDispatcher is now deprecated. If you were extending this with your own dispatcher then you need to refactor to extend Symfony\Component\EventDispatcher\EventDispatcher instead.
Additionally the \Drupal\Core\DependencyInjection\Compiler\RegisterEventSubscribersPass compiler pass which registered event subscribers with the dispatcher has been altered to work with the Symfony dispatcher instead.
