Mark old methods as deprecated

[MAILPOET-3267]

lib/Models/SendingQueue.php

@@ -112,6 +112,7 @@ class SendingQueue extends Model {
     return (Helpers::isJson($this->meta) && is_string($this->meta)) ? json_decode($this->meta, true) : $this->meta;
   }
 
+  /** @deprecated \MailPoet\Newsletter\Sending\SendingQueuesRepository::isSubscriberProcessed */
   public function isSubscriberProcessed($subscriberId) {
     if (!empty($this->subscribers)
       && ScheduledTaskSubscriber::getTotalCount($this->taskId) === 0

lib/Tasks/Subscribers.php

@@ -20,6 +20,7 @@ class Subscribers {
     return ScheduledTaskSubscriber::where('task_id', $this->task->id);
   }
 
+  /** @deprecated \MailPoet\Newsletter\Sending\ScheduledTaskSubscribersRepository::isSubscriberProcessed */
   public function isSubscriberProcessed($subscriberId) {
     $subscriber = $this->getSubscribers()
       ->where('subscriber_id', $subscriberId)