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.

for (int i = 0; i < 100 && pindex != NULL; i++)
        {
            int32_t nExpectedVersion = ComputeBlockVersion(pindex->pprev, chainParams.GetConsensus());
            if (pindex->nVersion > VERSIONBITS_LAST_OLD_BLOCK_VERSION && (pindex->nVersion & ~nExpectedVersion) != 0)
                ++nUpgraded;
            pindex = pindex->pprev;
        }
        if (nUpgraded > 0)
            warningMessages.push_back(strprintf("%d of last 100 blocks have unexpected version", nUpgraded));
        if (nUpgraded > 100/2)
        {
            
            strMiscWarning = _("Warning: Unknown block versions being mined! It's possible unknown rules are in effect");
            if (!fWarned) {
                AlertNotify(strMiscWarning);
                fWarned = true;
            }
        }
    }
    LogPrintf("%s: new best=%s height=%d version=0x%08x log2_work=%.8g tx=%lu date='%s' progress=%f cache=%.1fMiB(%utx)", __func__,
      chainActive.Tip()->GetBlockHash().ToString(), chainActive.Height(), chainActive.Tip()->nVersion,
      log(chainActive.Tip()->nChainWork.getdouble())/log(2.0), (unsigned long)chainActive.Tip()->nChainTx,
      DateTimeStrFormat("%Y-%m-%d %H:%M:%S", chainActive.Tip()->GetBlockTime()),
      Checkpoints::GuessVerificationProgress(chainParams.Checkpoints(), chainActive.Tip()), pcoinsTip->DynamicMemoryUsage() * (1.0 / (1<<20)), pcoinsTip->GetCacheSize());
    if (!warningMessages.empty())
        LogPrintf(" warning='%s'", boost::algorithm::join(warningMessages, ", "));
    LogPrintf("\n");
}
bool static DisconnectTip(CValidationState& state, const CChainParams& chainparams, bool fBare = false)