`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 (!init) return SynchronizationState::POST_INIT;
    if (::fReindex) return SynchronizationState::INIT_REINDEX;
    return SynchronizationState::INIT_DOWNLOAD;
}
static bool NotifyHeaderTip(CChainState& chainstate) LOCKS_EXCLUDED(cs_main) {
    bool fNotify = false;
    bool fInitialBlockDownload = false;
    static CBlockIndex* pindexHeaderOld = nullptr;
    CBlockIndex* pindexHeader = nullptr;
    {
        LOCK(cs_main);
        pindexHeader = pindexBestHeader;
        if (pindexHeader != pindexHeaderOld) {
            fNotify = true;
            assert(std::addressof(::ChainstateActive()) == std::addressof(chainstate));
            fInitialBlockDownload = chainstate.IsInitialBlockDownload();
            pindexHeaderOld = pindexHeader;
        }
    }
    
    if (fNotify) {
        uiInterface.NotifyHeaderTip(GetSynchronizationState(fInitialBlockDownload), pindexHeader);
    }
    return fNotify;
}
static void LimitValidationInterfaceQueue() LOCKS_EXCLUDED(cs_main) {