Non GST Declaration is Compulsorry

No. It is not compulsory for the buyer to get a NOn-GST declaration from the supplier when buying goods or services from an unregisered GST entity and the buyer's turnover is under Rs.20 lakhs. 

If supplier tunover is exceeds 20 lakh and we not collected the declaration.

In that case what would happen?

No GST declaration is prescribed as a general requirement for every purchase from an unregistered supplier. The buyer should retain the supplier’s invoice or bill of supply/receipt and normal purchase evidence; a signed declaration confirming the supplier’s GST registration status may be kept as supporting documentation, especially if the supplier’s turnover or registration status is relevant to the transaction.

A supplier below the applicable registration threshold is not automatically unregistered: compulsory-registration provisions and the applicable threshold (which can vary by State and supply type) must also be considered. If the supplier is unregistered, the buyer generally pays no GST to that supplier and cannot claim input tax credit. Reverse charge applies only to notified categories, not to all purchases from unregistered persons.

For vendor onboarding, this status check should sit alongside invoice and purchase-record controls. Manual verification across suppliers and transactions is difficult; automation can reduce repetitive work while keeping tax treatment and exceptions under professional judgment. At Jir AI, our 2,000+ AI agents and 650+ integrations streamline GST, ITR, TDS, TCS and other compliance workflows, helping reduce manual workload by up to 70%. WhatsApp us to explore a workflow for your practice: https://wa.me/message/3HVOZ7HIY4ESC1