`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 (!fCheckBlockIndex) {
        return;
    }
    LOCK(cs_main);
    
    
    
    if (m_chain.Height() < 0) {
        assert(m_blockman.m_block_index.size() <= 1);
        return;
    }
    
    std::multimap<CBlockIndex*,CBlockIndex*> forward;
    for (const std::pair<const uint256, CBlockIndex*>& entry : m_blockman.m_block_index) {
        forward.insert(std::make_pair(entry.second->pprev, entry.second));
    }
    assert(forward.size() == m_blockman.m_block_index.size());
    std::pair<std::multimap<CBlockIndex*,CBlockIndex*>::iterator,std::multimap<CBlockIndex*,CBlockIndex*>::iterator> rangeGenesis = forward.equal_range(nullptr);
    CBlockIndex *pindex = rangeGenesis.first->second;
    rangeGenesis.first++;
    assert(rangeGenesis.first == rangeGenesis.second);