`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);
    
    bool needs_init = fReindex;
    if (!fReindex) {
        bool ret = ActiveChainstate().LoadBlockIndexDB(chainparams);
        if (!ret) return false;
        needs_init = m_blockman.m_block_index.empty();
    }
    if (needs_init) {
        
        
        
        
        
        LogPrintf("Initializing databases...\n");
    }
    return true;
}
bool CChainState::LoadGenesisBlock(const CChainParams& chainparams)