`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.
CScriptCheck(const CTxOut& outIn, const CTransaction& txToIn, unsigned int nInIn, unsigned int nFlagsIn, bool cacheIn, PrecomputedTransactionData* txdataIn) :
        m_tx_out(outIn), ptxTo(&txToIn), nIn(nInIn), nFlags(nFlagsIn), cacheStore(cacheIn), error(SCRIPT_ERR_UNKNOWN_ERROR), txdata(txdataIn) { }
    bool operator()();
    void swap(CScriptCheck &check) {
        std::swap(ptxTo, check.ptxTo);
        std::swap(m_tx_out, check.m_tx_out);
        std::swap(nIn, check.nIn);
        std::swap(nFlags, check.nFlags);
        std::swap(cacheStore, check.cacheStore);
        std::swap(error, check.error);
        std::swap(txdata, check.txdata);
    }
    ScriptError GetScriptError() const { return error; }
};
void InitScriptExecutionCache();
bool CheckBlock(const CBlock& block, BlockValidationState& state, const Consensus::Params& consensusParams, bool fCheckPOW = true, bool fCheckMerkleRoot = true);
bool TestBlockValidity(BlockValidationState& state,
                       const CChainParams& chainparams,
                       CChainState& chainstate,
                       const CBlock& block,
                       CBlockIndex* pindexPrev,
                       bool fCheckPOW = true,
                       bool fCheckMerkleRoot = true) EXCLUSIVE_LOCKS_REQUIRED(cs_main);
/** Check whether witness commitments are required for a block, and whether to enforce NULLDUMMY (BIP 147) rules.
 *  Note that transaction witness validation rules are always enforced when P2SH is enforced. */
bool IsWitnessEnabled(const CBlockIndex* pindexPrev, const Consensus::Params& params);
void UpdateUncommittedBlockStructures(CBlock& block, const CBlockIndex* pindexPrev, const Consensus::Params& consensusParams);
std::vector<unsigned char> GenerateCoinbaseCommitment(CBlock& block, const CBlockIndex* pindexPrev, const Consensus::Params& consensusParams);
class CVerifyDB {