`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.
++failed;
                    }
                }
            } else {
                ++expired;
            }
            if (ShutdownRequested())
                return false;
        }
        std::map<uint256, CAmount> mapDeltas;
        file >> mapDeltas;
        for (const auto& i : mapDeltas) {
            pool.PrioritiseTransaction(i.first, i.second);
        }
        std::set<uint256> unbroadcast_txids;
        file >> unbroadcast_txids;
        unbroadcast = unbroadcast_txids.size();
        for (const auto& txid : unbroadcast_txids) {
            
            
            if (pool.get(txid) != nullptr) pool.AddUnbroadcastTx(txid);
        }
    } catch (const std::exception& e) {
        LogPrintf("Failed to deserialize mempool data on disk: %s. Continuing anyway.\n", e.what());
        return false;
    }
    LogPrintf("Imported mempool transactions from disk: %i succeeded, %i failed, %i expired, %i already there, %i waiting for initial broadcast\n", count, failed, expired, already_there, unbroadcast);
    return true;
}
bool DumpMempool(const CTxMemPool& pool, FopenFn mockable_fopen_function, bool skip_file_commit)