According to the information I found, the developer is not required to pay GST under RCM if the flat sale is after the issuance of completion certificate. This is because the sale of completed flats is neither a supply of goods nor a supply of services under GST, and hence no GST is applicable on such transactions123. However, the developer may still have to pay GST under RCM on other supplies, such as TDR, FSI, or works contract services, depending on the conditions and rates specified by the CBIC456. The proportionate reversal of input tax credit is only applicable if the developer opts to pay tax at full rate (8%/12%) after availing ITC, and not if the developer opts for the new scheme of 1%/5% without ITC4. I hope this clarifies your query.