You need to update to IRCC if the gap is more than 150 days.
"1/4 Hi. A deferral in start-date is between you and your DLI. It shouldn’t affect your study permit application, though if you defer your start-date by 150 days or more, you’ll need to send us a new acceptance letter from your DLI."
This is what IRCC replied on twitter for a similar query.