`RewindBlockIndex` was made to allow nodes upgrading to new consensus rules post-segwit fork to 
download new consensus data (i.e witnesses) without having to go through a whole IBD. It has been pointed than now really few pre-segwit nodes care to upgrade and that this logic may be irrelevant.

See https://github.com/bitcoin/bitcoin/pull/17737#discussion_r358514335 in review  of #17737 for more context pointers.
const int maxDescendantsToVisit = 100;
        for (const auto& mi : setIterConflicting) {
            
            
            
            
            
            
            
            
            
            
            
            
            
            
            CFeeRate oldFeeRate(mi->GetModifiedFee(), mi->GetTxSize());
            if (newFeeRate <= oldFeeRate)
            {
                return state.Invalid(TxValidationResult::TX_MEMPOOL_POLICY, "insufficient fee",
                        strprintf("rejecting replacement %s; new feerate %s <= old feerate %s",
                            hash.ToString(),
                            newFeeRate.ToString(),
                            oldFeeRate.ToString()));
            }
            for (const CTxIn &txin : mi->GetTx().vin)
            {
                setConflictsParents.insert(txin.prevout.hash);
            }
            nConflictingCount += mi->GetCountWithDescendants();