Deprecate GetModelIsPrivate and its usages

src/Discussion/Discussion.php

@@ -110,6 +110,10 @@ class Discussion extends AbstractModel
             Notification::whereSubject($discussion)->delete();
         });
 
+        /**
+         * @deprecated beta 15, remove beta 16
+         * When needed, the `Flarum\Discussion\Event\Saving` event should be listened to directly.
+         */
         static::saving(function (self $discussion) {
             $event = new GetModelIsPrivate($discussion);
 

src/Event/GetModelIsPrivate.php

@@ -13,6 +13,9 @@ use Flarum\Database\AbstractModel;
 
 /**
  * Determine whether or not a model should be marked as `is_private`.
+ *
+ * @deprecated beta 15, remove beta 16
+ * When needed, the `Flarum\Discussion\Event\Saving` event should be listened to directly.
  */
 class GetModelIsPrivate
 {

src/Post/Post.php

@@ -96,6 +96,10 @@ class Post extends AbstractModel
             $post->discussion->save();
         });
 
+        /**
+         * @deprecated beta 15, remove beta 16
+         * When needed, the `Flarum\Discussion\Event\Saving` event should be listened to directly.
+         */
         static::saving(function (self $post) {
             $event = new GetModelIsPrivate($post);