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.

if (ratio != 0) {
        mempool.setSanityCheck(1.0 / ratio);
    }
    fCheckBlockIndex = GetBoolArg("-checkblockindex", chainparams.DefaultConsistencyChecks());
    fCheckpointsEnabled = GetBoolArg("-checkpoints", DEFAULT_CHECKPOINTS_ENABLED);
    
    int64_t nMempoolSizeMax = GetArg("-maxmempool", DEFAULT_MAX_MEMPOOL_SIZE) * 1000000;
    int64_t nMempoolSizeMin = GetArg("-limitdescendantsize", DEFAULT_DESCENDANT_SIZE_LIMIT) * 1000 * 40;
    if (nMempoolSizeMax < 0 || nMempoolSizeMax < nMempoolSizeMin)
        return InitError(strprintf(_("-maxmempool must be at least %d MB"), std::ceil(nMempoolSizeMin / 1000000.0)));
    
    nScriptCheckThreads = GetArg("-par", DEFAULT_SCRIPTCHECK_THREADS);
    if (nScriptCheckThreads <= 0)
        nScriptCheckThreads += GetNumCores();
    if (nScriptCheckThreads <= 1)
        nScriptCheckThreads = 0;
    else if (nScriptCheckThreads > MAX_SCRIPTCHECK_THREADS)
        nScriptCheckThreads = MAX_SCRIPTCHECK_THREADS;
    fServer = GetBoolArg("-server", false);
    
    int64_t nSignedPruneTarget = GetArg("-prune", 0) * 1024 * 1024;
    if (nSignedPruneTarget < 0) {
        return InitError(_("Prune cannot be configured with a negative value."));
    }
    nPruneTarget = (uint64_t) nSignedPruneTarget;
    if (nPruneTarget) {
        if (nPruneTarget < MIN_DISK_SPACE_FOR_BLOCK_FILES) {
            return InitError(strprintf(_("Prune configured below the minimum of %d MiB.  Please use a higher number."), MIN_DISK_SPACE_FOR_BLOCK_FILES / 1024 / 1024));
        }
        LogPrintf("Prune configured to target %uMiB on disk for block and undo files.\n", nPruneTarget / 1024 / 1024);