There is a previous thread about this, from 2014, which was recently archived: Notification for user approval even after user is approved
However, this is still an issue which I would like to raise attention to.
When the user is approved, all other admins still get a PM marked as unread asking them to do something, while there is nothing to do. This is quite annoying. I would not like to disable this feature entirely, but mark the message as read for the other admins when one of the admins approves the user.