CRM_Utils_System_Drupal8::setMessage() is fine for Drupal 8.
Set up routing for civicrm/logout.
Update CRM_Utils_System_Drupal8::isUserLoggedIn() to Drupal 8.
Synchronize the Drupal and Civicrm user on ever CiviCRM request.

I'm unsure why this is required, but this is in keeping with the Drupal 7 implementation.