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.

{
        
        
        
        
        
        
        
        
        
        
        
        
        if (scriptPubKey.IsUnspendable())
            return 0;
        size_t nSize = GetSerializeSize(SER_DISK, 0);
        int witnessversion = 0;
        std::vector<unsigned char> witnessprogram;
        if (scriptPubKey.IsWitnessProgram(witnessversion, witnessprogram)) {
            
            
            nSize += (32 + 4 + 1 + (107 / WITNESS_SCALE_FACTOR) + 4);
        } else {
            nSize += (32 + 4 + 1 + 107 + 4); 
        }
        return 3 * minRelayTxFee.GetFee(nSize);
    }
    bool IsDust(const CFeeRate &minRelayTxFee) const