`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.
{
    AssertLockHeld(cs_main);
    assert(active_chain_tip); 
    assert(std::addressof(*::ChainActive().Tip()) == std::addressof(*active_chain_tip));
    
    
    
    
    
    
    flags = std::max(flags, 0);
    
    
    
    
    
    
    const int nBlockHeight = active_chain_tip->nHeight + 1;
    
    
    
    
    
    const int64_t nBlockTime = (flags & LOCKTIME_MEDIAN_TIME_PAST)
                             ? active_chain_tip->GetMedianTimePast()
                             : GetAdjustedTime();
    return IsFinalTx(tx, nBlockHeight, nBlockTime);
}
bool TestLockPointValidity(CChain& active_chain, const LockPoints* lp)