The user of-course can already implicitly work out this information from comparing the fee-rate with the actual fee.

I think that it would be useful for people not wanting to do this arithmetic in their head.

{
        for (size_t n = 0; n < vtxinwit.size(); n++) {
            READWRITE(vtxinwit[n]);
        }
        if (IsNull()) {
            
            throw std::ios_base::failure("Superfluous witness record");
        }
    }
};
struct CMutableTransaction;
/**
 * Basic transaction serialization format:
 * - int32_t nVersion
 * - std::vector<CTxIn> vin
 * - std::vector<CTxOut> vout
 * - uint32_t nLockTime
 *
 * Extended transaction serialization format:
 * - int32_t nVersion
 * - unsigned char dummy = 0x00
 * - unsigned char flags (!= 0)
 * - std::vector<CTxIn> vin
 * - std::vector<CTxOut> vout
 * - if (flags & 1):
 *   - CTxWitness wit;
 * - uint32_t nLockTime
 */
template<typename Stream, typename Operation, typename TxType>
inline void SerializeTransaction(TxType& tx, Stream& s, Operation ser_action, int nType, int nVersion) {