The due date for filing GSTR-9 for the financial year 2017-18 was extended multiple times. As per the latest updates before my last training cut-off in October 2023, the final due date for filing GSTR-9 for FY 2017-18 was October 31, 2019. However, it's important to check the latest notifications from the GST portal or consult with a tax professional for any updates or changes in deadlines.