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.

break;
                }
                
                if (fTxIndex != GetBoolArg("-txindex", DEFAULT_TXINDEX)) {
                    strLoadError = _("You need to rebuild the database using -reindex-chainstate to change -txindex");
                    break;
                }
                
                
                if (fHavePruned && !fPruneMode) {
                    strLoadError = _("You need to rebuild the database using -reindex to go back to unpruned mode.  This will redownload the entire blockchain");
                    break;
                }
                if (!fReindex && chainActive.Tip() != NULL) {
                    uiInterface.InitMessage(_("Rewinding blocks..."));
                    if (!RewindBlockIndex(chainparams)) {
                        strLoadError = _("Unable to rewind the database to a pre-fork state. You will need to redownload the blockchain");
                        break;
                    }
                }
                uiInterface.InitMessage(_("Verifying blocks..."));
                if (fHavePruned && GetArg("-checkblocks", DEFAULT_CHECKBLOCKS) > MIN_BLOCKS_TO_KEEP) {
                    LogPrintf("Prune: pruned datadir may not have more than %d blocks; only checking available blocks",
                        MIN_BLOCKS_TO_KEEP);
                }
                {
                    LOCK(cs_main);
                    CBlockIndex* tip = chainActive.Tip();
                    RPCNotifyBlockChange(true, tip);
                    if (tip && tip->nTime > GetAdjustedTime() + 2 * 60 * 60) {