`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.
nSigOpsCost += GetTransactionSigOpCost(tx, view, flags);
        if (nSigOpsCost > MAX_BLOCK_SIGOPS_COST) {
            LogPrintf("ERROR: ConnectBlock(): too many sigops\n");
            return state.Invalid(BlockValidationResult::BLOCK_CONSENSUS, "bad-blk-sigops");
        }
        if (!tx.IsCoinBase())
        {
            std::vector<CScriptCheck> vChecks;
            bool fCacheResults = fJustCheck; 
            TxValidationState tx_state;
            if (fScriptChecks && !CheckInputScripts(tx, tx_state, view, flags, fCacheResults, fCacheResults, txsdata[i], g_parallel_script_checks ? &vChecks : nullptr)) {
                
                state.Invalid(BlockValidationResult::BLOCK_CONSENSUS,
                              tx_state.GetRejectReason(), tx_state.GetDebugMessage());
                return error("ConnectBlock(): CheckInputScripts on %s failed with %s",
                    tx.GetHash().ToString(), state.ToString());
            }
            control.Add(vChecks);
        }
        CTxUndo undoDummy;
        if (i > 0) {
            blockundo.vtxundo.push_back(CTxUndo());
        }
        UpdateCoins(tx, view, i == 0 ? undoDummy : blockundo.vtxundo.back(), pindex->nHeight);
    }
    int64_t nTime3 = GetTimeMicros(); nTimeConnect += nTime3 - nTime2;