`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);
    
    uint256 hash = block.GetHash();
    BlockMap::iterator it = m_block_index.find(hash);
    if (it != m_block_index.end())
        return it->second;
    
    CBlockIndex* pindexNew = new CBlockIndex(block);
    
    
    
    pindexNew->nSequenceId = 0;
    BlockMap::iterator mi = m_block_index.insert(std::make_pair(hash, pindexNew)).first;