`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.
{
    std::vector<COutPoint> coins_to_uncache;
    MemPoolAccept::ATMPArgs args { chainparams, nAcceptTime, bypass_limits, coins_to_uncache, test_accept };
    assert(std::addressof(::ChainstateActive()) == std::addressof(active_chainstate));
    const MempoolAcceptResult result = MemPoolAccept(pool, active_chainstate).AcceptSingleTransaction(tx, args);
    if (result.m_result_type != MempoolAcceptResult::ResultType::VALID) {
        
        
        
        
        for (const COutPoint& hashTx : coins_to_uncache)
            active_chainstate.CoinsTip().Uncache(hashTx);
    }
    
    BlockValidationState state_dummy;
    active_chainstate.FlushStateToDisk(chainparams, state_dummy, FlushStateMode::PERIODIC);
    return result;
}
MempoolAcceptResult AcceptToMemoryPool(CChainState& active_chainstate, CTxMemPool& pool, const CTransactionRef& tx,
                                       bool bypass_limits, bool test_accept)
{
    assert(std::addressof(::ChainstateActive()) == std::addressof(active_chainstate));
    return AcceptToMemoryPoolWithTime(Params(), pool, active_chainstate, tx, GetTime(), bypass_limits, test_accept);
}
CTransactionRef GetTransaction(const CBlockIndex* const block_index, const CTxMemPool* const mempool, const uint256& hash, const Consensus::Params& consensusParams, uint256& hashBlock)