`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.
{
    LOCK(::cs_main);
    assert(m_active_chainstate);
    return *m_active_chainstate;
}
bool ChainstateManager::IsSnapshotActive() const
{
    LOCK(::cs_main);
    return m_snapshot_chainstate && m_active_chainstate == m_snapshot_chainstate.get();
}
CChainState& ChainstateManager::ValidatedChainstate() const