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.

bool addUnchecked(const uint256& hash, const CTxMemPoolEntry &entry, bool fCurrentEstimate = true);
    bool addUnchecked(const uint256& hash, const CTxMemPoolEntry &entry, setEntries &setAncestors, bool fCurrentEstimate = true);
    void removeRecursive(const CTransaction &tx, std::vector<std::shared_ptr<const CTransaction>>* removed = NULL);
    void removeForReorg(const CCoinsViewCache *pcoins, unsigned int nMemPoolHeight, int flags);
    void removeConflicts(const CTransaction &tx, std::vector<std::shared_ptr<const CTransaction>>* removed = NULL);
    void removeForBlock(const std::vector<CTransaction>& vtx, unsigned int nBlockHeight,
                        std::vector<std::shared_ptr<const CTransaction>>* conflicts = NULL, bool fCurrentEstimate = true);
    void clear();
    void _clear(); 
    bool CompareDepthAndScore(const uint256& hasha, const uint256& hashb);
    void queryHashes(std::vector<uint256>& vtxid);
    void pruneSpent(const uint256& hash, CCoins &coins);
    unsigned int GetTransactionsUpdated() const;
    void AddTransactionsUpdated(unsigned int n);
    /**
     * Check that none of this transactions inputs are in the mempool, and thus
     * the tx is not dependent on other mempool transactions to be included in a block.
     */
    bool HasNoInputsOf(const CTransaction& tx) const;
    
    void PrioritiseTransaction(const uint256 hash, const std::string strHash, double dPriorityDelta, const CAmount& nFeeDelta);
    void ApplyDeltas(const uint256 hash, double &dPriorityDelta, CAmount &nFeeDelta) const;