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 (pindexBestForkTip && chainActive.Height() - pindexBestForkTip->nHeight >= 72)
        pindexBestForkTip = NULL;
    if (pindexBestForkTip || (pindexBestInvalid && pindexBestInvalid->nChainWork > chainActive.Tip()->nChainWork + (GetBlockProof(*chainActive.Tip()) * 6)))
    {
        if (!fLargeWorkForkFound && pindexBestForkBase)
        {
            std::string warning = std::string("'Warning: Large-work fork detected, forking after block ") +
                pindexBestForkBase->phashBlock->ToString() + std::string("'");
            AlertNotify(warning);
        }
        if (pindexBestForkTip && pindexBestForkBase)
        {
            LogPrintf("%s: Warning: Large valid fork found\n  forking the chain at height %d (%s)\n  lasting to height %d (%s).\nChain state database corruption likely.\n", __func__,
                   pindexBestForkBase->nHeight, pindexBestForkBase->phashBlock->ToString(),
                   pindexBestForkTip->nHeight, pindexBestForkTip->phashBlock->ToString());
            fLargeWorkForkFound = true;
        }
        else
        {
            LogPrintf("%s: Warning: Found invalid chain at least ~6 blocks longer than our best chain.\nChain state database corruption likely.\n", __func__);
            fLargeWorkInvalidChainFound = true;
        }
    }
    else
    {
        fLargeWorkForkFound = false;
        fLargeWorkInvalidChainFound = false;
    }
}
void CheckForkWarningConditionsOnNewFork(CBlockIndex* pindexNewForkTip)