The Central Board of Direct Taxes vide Notification No. 31/2019 dated 31st March 2019 has extended the last date for linking of PAN and Aadhar till 30th September 2019. However, after 01st April 2019, it is mandatory to quote Aadhar Number while filing the income tax return.
https://www.incometaxindia.gov.in/communications/notification/notification_31_2019.pdf