Currently the mempool isn't persisted across restarts. The historical argument against doing this is that it would make transactions immortal. Expiration now addresses that issue.

Without preserving the mempool node behavior changes drastically across restarts which complicates production use (e.g. mining overly small/low fee blocks) as well as development testing. The lack of retention also causes the loss of txn priority modifications made with prioritisetransaction which makes the API less reliable.

(GetBlockProofEquivalentTime(*pindexBestHeader, *mi->second, *pindexBestHeader, consensusParams) < nOneMonth);
                        if (!send) {
                            LogPrintf("%s: ignoring request from peer=%i for old block that isn't in the main chain\n", __func__, pfrom->GetId());
                        }
                    }
                }
                
                
                static const int nOneWeek = 7 * 24 * 60 * 60; 
                if (send && connman.OutboundTargetReached(true) && ( ((pindexBestHeader != NULL) && (pindexBestHeader->GetBlockTime() - mi->second->GetBlockTime() > nOneWeek)) || inv.type == MSG_FILTERED_BLOCK) && !pfrom->fWhitelisted)
                {
                    LogPrint("net", "historical block serving limit reached, disconnect peer=%d\n", pfrom->GetId());
                    
                    pfrom->fDisconnect = true;
                    send = false;
                }
                
                
                if (send && (mi->second->nStatus & BLOCK_HAVE_DATA))
                {
                    
                    CBlock block;
                    if (!ReadBlockFromDisk(block, (*mi).second, consensusParams))
                        assert(!"cannot load block from disk");
                    if (inv.type == MSG_BLOCK)
                        pfrom->PushMessageWithFlag(SERIALIZE_TRANSACTION_NO_WITNESS, NetMsgType::BLOCK, block);
                    else if (inv.type == MSG_WITNESS_BLOCK)
                        pfrom->PushMessage(NetMsgType::BLOCK, block);
                    else if (inv.type == MSG_FILTERED_BLOCK)
                    {