`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 CChainParams& chainparams,
                       CChainState& chainstate,
                       const CBlock& block,
                       CBlockIndex* pindexPrev,
                       bool fCheckPOW,
                       bool fCheckMerkleRoot)
{
    AssertLockHeld(cs_main);
    assert(std::addressof(::ChainstateActive()) == std::addressof(chainstate));
    assert(pindexPrev && pindexPrev == chainstate.m_chain.Tip());
    CCoinsViewCache viewNew(&chainstate.CoinsTip());
    uint256 block_hash(block.GetHash());