`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.
int ActiveHeight() const { return ActiveChain().Height(); }
    CBlockIndex* ActiveTip() const { return ActiveChain().Tip(); }
    BlockMap& BlockIndex() EXCLUSIVE_LOCKS_REQUIRED(::cs_main)
    {
        return m_blockman.m_block_index;
    }
    bool IsSnapshotActive() const;
    std::optional<uint256> SnapshotBlockhash() const;
    
    bool IsSnapshotValidated() const { return m_snapshot_validated; }
    
    
    bool IsBackgroundIBD(CChainState* chainstate) const;
    
    
    
    
    CChainState& ValidatedChainstate() const;
    CChain& ValidatedChain() const { return ValidatedChainstate().m_chain; }