Thank-you so much for putting out the latest D9 compatible release! I am testing it and I found that there are some changes in MessageNotifierBase that will make this module cause a WSOD and the error:
TypeError: Argument 7 passed to Drupal\message_notify\Plugin\Notifier\MessageNotifierBase::__construct() must be an instance of Drupal\Core\Session\AccountSwitcher, instance of Drupal\message\Entity\Message given
Closed: cannot reproduce
1.0
Code
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