Is GSTIN Mandatory in this case?

Persons engaged exclusively in the supply of exempted goods or services or both are the ones not liable to obtain registration. Sec. 23(1)(a) of the CGST Act.

Further, as per Sec. 24(i) of the CGST Act, persons making any inter-State taxable supply are required to obtain registration compulsorily under this Act. But the supply should be taxable. So, a person engaged exclusively in the supply of exempted goods is exempted from obtaining registration.

It should be noted that, such person will be required to obtain registration, if he procures the notified goods /services (covered u/s 9(3) of the CGST Act, i.e. RCM), even though sec. 23 gives immunity to such persons from registration.