Resolve #3490037 "Min drupal 10.3"

Closes #3490037

src/Mapper/PrivateMessageMapper.php

@@ -2,6 +2,8 @@
 
 namespace Drupal\private_message\Mapper;
 
+@trigger_error(__CLASS__ . ' is deprecated in private_message:4.0.0 and is removed from private_message:5.0.0. No replacement is provided. See https://www.drupal.org/node/3490530', E_USER_DEPRECATED);
+
 use Drupal\Core\Database\Connection;
 use Drupal\Core\Database\Query\SelectInterface;
 use Drupal\Core\Session\AccountProxyInterface;
@@ -10,6 +12,11 @@ use Drupal\user\UserInterface;
 
 /**
  * Interface for the Private Message Mapper class.
+ *
+ * @deprecated in private_message:4.0.0 and is removed from
+ *   private_message:5.0.0. No replacement is provided.
+ *
+ * @see https://www.drupal.org/node/3490530
  */
 class PrivateMessageMapper implements PrivateMessageMapperInterface {