Shift+close without sending email notification to the ticket creator, this functionality does not work. The notification still sends email that the ticket was closed

The reason for this could be because of an Observer rule being triggered and separate email being sent out. Shift+Close will stop only the Default "Agent closes a ticket" notification that you set from Admin--> Email Notifications--> Requester Notifications. However, the Observer rules will be based on the action that is performed.