`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.
if (::GetSerializeSize(tx, PROTOCOL_VERSION | SERIALIZE_TRANSACTION_NO_WITNESS) < MIN_STANDARD_TX_NONWITNESS_SIZE)
        return state.Invalid(TxValidationResult::TX_NOT_STANDARD, "tx-size-small");
    
    
    
    assert(std::addressof(::ChainActive()) == std::addressof(m_active_chainstate.m_chain));
    if (!CheckFinalTx(m_active_chainstate.m_chain.Tip(), tx, STANDARD_LOCKTIME_VERIFY_FLAGS))
        return state.Invalid(TxValidationResult::TX_PREMATURE_SPEND, "non-final");
    
    if (m_pool.exists(hash)) {
        return state.Invalid(TxValidationResult::TX_CONFLICT, "txn-already-in-mempool");
    }
    
    for (const CTxIn &txin : tx.vin)
    {
        const CTransaction* ptxConflicting = m_pool.GetConflictTx(txin.prevout);
        if (ptxConflicting) {
            if (!setConflicts.count(ptxConflicting->GetHash()))
            {