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.

uiInterface.InitMessage(_("Loading block index..."));
        nStart = GetTimeMillis();
        do {
            try {
                UnloadBlockIndex();
                delete pcoinsTip;
                delete pcoinsdbview;
                delete pcoinscatcher;
                delete pblocktree;
                pblocktree = new CBlockTreeDB(nBlockTreeDBCache, false, fReindex);
                pcoinsdbview = new CCoinsViewDB(nCoinDBCache, false, fReindex || fReindexChainState);
                pcoinscatcher = new CCoinsViewErrorCatcher(pcoinsdbview);
                pcoinsTip = new CCoinsViewCache(pcoinscatcher);
                if (fReindex) {
                    pblocktree->WriteReindexing(true);
                    
                    if (fPruneMode)
                        CleanupBlockRevFiles();
                }
                if (!LoadBlockIndex()) {
                    strLoadError = _("Error loading block database");
                    break;
                }
                
                
                if (!mapBlockIndex.empty() && mapBlockIndex.count(chainparams.GetConsensus().hashGenesisBlock) == 0)
                    return InitError(_("Incorrect or no genesis block found. Wrong datadir for network?"));
                
                if (!InitBlockIndex(chainparams)) {
                    strLoadError = _("Error initializing block database");