`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);
            LOCK(m_mempool.cs); 
            CBlockIndex* starting_tip = m_chain.Tip();
            bool blocks_connected = false;
            do {
                
                
                ConnectTrace connectTrace; 
                if (pindexMostWork == nullptr) {
                    pindexMostWork = FindMostWorkChain();
                }
                
                if (pindexMostWork == nullptr || pindexMostWork == m_chain.Tip()) {
                    break;
                }
                bool fInvalidFound = false;
                std::shared_ptr<const CBlock> nullBlockPtr;
                if (!ActivateBestChainStep(state, chainparams, pindexMostWork, pblock && pblock->GetHash() == pindexMostWork->GetBlockHash() ? pblock : nullBlockPtr, fInvalidFound, connectTrace)) {
                    
                    return false;
                }
                blocks_connected = true;
                if (fInvalidFound) {
                    
                    pindexMostWork = nullptr;
                }
                pindexNewTip = m_chain.Tip();
                for (const PerBlockConnectTrace& trace : connectTrace.GetBlocksConnected()) {
                    assert(trace.pblock && trace.pindex);
                    GetMainSignals().BlockConnected(trace.pblock, trace.pindex);