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.

void ClearPrioritisation(const uint256 hash);
public:
    /** Remove a set of transactions from the mempool.
     *  If a transaction is in this set, then all in-mempool descendants must
     *  also be in the set, unless this transaction is being removed for being
     *  in a block.
     *  Set updateDescendants to true when removing a tx that was in a block, so
     *  that any in-mempool descendants have their ancestor state updated.
     */
    void RemoveStaged(setEntries &stage, bool updateDescendants);
    /** When adding transactions from a disconnected block back to the mempool,
     *  new mempool entries may have children in the mempool (which is generally
     *  not the case when otherwise adding transactions).
     *  UpdateTransactionsFromBlock() will find child transactions and update the
     *  descendant state for each transaction in hashesToUpdate (excluding any
     *  child transactions present in hashesToUpdate, which are already accounted
     *  for).  Note: hashesToUpdate should be the set of transactions from the
     *  disconnected block that have been accepted back into the mempool.
     */
    void UpdateTransactionsFromBlock(const std::vector<uint256> &hashesToUpdate);
    /** Try to calculate all in-mempool ancestors of entry.
     *  (these are all calculated including the tx itself)
     *  limitAncestorCount = max number of ancestors
     *  limitAncestorSize = max size of ancestors
     *  limitDescendantCount = max number of descendants any ancestor can have
     *  limitDescendantSize = max size of descendants any ancestor can have
     *  errString = populated with error reason if any limits are hit
     *  fSearchForParents = whether to search a tx's vin for in-mempool parents, or
     *    look up parents from mapLinks. Must be true for entries not in the mempool
     */