`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.
{
    const CBlock& block = *pblock;
    if (fNewBlock) *fNewBlock = false;
    AssertLockHeld(cs_main);
    CBlockIndex *pindexDummy = nullptr;
    CBlockIndex *&pindex = ppindex ? *ppindex : pindexDummy;
    bool accepted_header = m_blockman.AcceptBlockHeader(block, state, chainparams, &pindex);
    CheckBlockIndex(chainparams.GetConsensus());