Do some tasks after a simplenews newsletter has been sent I need to unsubscribe users from a specific simplenews newsletter category after a newsletter of that category has been sent. The best way is to do this action after updating sent status of newsletter. but there are not a hook or event to do this. What is your opinion to implement this task?
The best way would be patch simplenews to offer more events.

Second best would be to override simplenews via a custom module to offer more events.

There are other options but those are the best following the Drupal way.