Use case: how to send a message to all users? about message HOT 2 OPEN

If you are using the message_subscribe_ui module, the subscribers are gathered for you when you call

 $subscribers = \Drupal::service('message_subscribe.subscribers');
  $subscribers->sendMessage($node, $message);

They are defined by flags on the content (eg, subscriptions to nodes are one flag, subscriptions to terms another.)

The message_subscribe_email module can be used either as an example (it builds on the message_subscribe_ui module.), or used as-is to achieve subscriptions.

from message.

In your case of wanting to notify all users, you could skip both the message_subscribe_ui and the message_subscribe_email modules and implement your own hook_message_subscribe_get_subscribers to return all users on the site.

from message.