IN my opinion it's supply of Services which is Exempted. (As per Notification 12/2017 CT (R) )
As per the Defination of Services u/s 2(102) of CGST Act : means anything other than goods, money & securities but ACTIVITIES RELATING TO USE OF MONEY ............
Bank pay Interest in lieu of Deposit.
Customer keeps money by depositing in bank & receiving Interest for that.