`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.
{
    AssertLockNotHeld(cs_main);
    assert(std::addressof(::ChainstateActive()) == std::addressof(ActiveChainstate()));
    {
        CBlockIndex *pindex = nullptr;
        if (fNewBlock) *fNewBlock = false;
        BlockValidationState state;
        
        
        LOCK(cs_main);
        
        
        bool ret = CheckBlock(*pblock, state, chainparams.GetConsensus());
        if (ret) {
            
            ret = ActiveChainstate().AcceptBlock(pblock, state, chainparams, &pindex, fForceProcessing, nullptr, fNewBlock);
        }
        if (!ret) {
            GetMainSignals().BlockChecked(*pblock, state);
            return error("%s: AcceptBlock FAILED (%s)", __func__, state.ToString());
        }
    }
    NotifyHeaderTip(ActiveChainstate());
    BlockValidationState state; 
    if (!ActiveChainstate().ActivateBestChain(state, chainparams, pblock))
        return error("%s: ActivateBestChain failed (%s)", __func__, state.ToString());
    return true;
}
bool TestBlockValidity(BlockValidationState& state,