`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.
assert(std::addressof(::ChainstateActive()) == std::addressof(*this));
                      if (AcceptBlock(pblock, state, chainparams, nullptr, true, dbp, nullptr)) {
                          nLoaded++;
                      }
                      if (state.IsError()) {
                          break;
                      }
                    } else if (hash != chainparams.GetConsensus().hashGenesisBlock && pindex->nHeight % 1000 == 0) {
                      LogPrint(BCLog::REINDEX, "Block Import: already had block %s at height %d\n", hash.ToString(), pindex->nHeight);
                    }
                }
                
                if (hash == chainparams.GetConsensus().hashGenesisBlock) {
                    BlockValidationState state;
                    assert(std::addressof(::ChainstateActive()) == std::addressof(*this));
                    if (!ActivateBestChain(state, chainparams, nullptr)) {
                        break;
                    }
                }
                assert(std::addressof(::ChainstateActive()) == std::addressof(*this));
                NotifyHeaderTip(*this);
                
                std::deque<uint256> queue;
                queue.push_back(hash);
                while (!queue.empty()) {
                    uint256 head = queue.front();
                    queue.pop_front();
                    std::pair<std::multimap<uint256, FlatFilePos>::iterator, std::multimap<uint256, FlatFilePos>::iterator> range = mapBlocksUnknownParent.equal_range(head);
                    while (range.first != range.second) {
                        std::multimap<uint256, FlatFilePos>::iterator it = range.first;