Dear Sir
In terms of Notification No.13/2017-Central Tax [Rate] dated 28/06/2017,the person who pays or is liable to pay freight for the transportation of goods by road in goods carriage, located in the taxable territory shall be treated as the person who receives the service for the purpose of paying tax under RCM. So the liability to pay tax under RCM is irrespective of whether the goods are taxable or exempted. It's just for the transportation of "goods".
However such RCM liability is subject to following rider, with effect from 18/07/2022:
Provided further that nothing contained in this entry shall apply where, -
i. the supplier has taken registration under the GST Act, 2017 and exercised the option to pay tax on the services of GTA in relation to transport of goods supplied by him under forward charge; and
ii. the supplier has issued a tax invoice to the recipient charging GST at the applicable rates and has made a declaration as prescribed in Annexure III on such invoice issued by him.