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 (showDebug)
        strUsage += HelpMessageOpt("-acceptnonstdtxn", strprintf("Relay and mine \"non-standard\" transactions (%sdefault: %u)", "testnet/regtest only; ", !Params(CBaseChainParams::TESTNET).RequireStandard()));
    strUsage += HelpMessageOpt("-bytespersigop", strprintf(_("Equivalent bytes per sigop in transactions for relay and mining (default: %u)"), DEFAULT_BYTES_PER_SIGOP));
    strUsage += HelpMessageOpt("-datacarrier", strprintf(_("Relay and mine data carrier transactions (default: %u)"), DEFAULT_ACCEPT_DATACARRIER));
    strUsage += HelpMessageOpt("-datacarriersize", strprintf(_("Maximum size of data in data carrier transactions we relay and mine (default: %u)"), MAX_OP_RETURN_RELAY));
    strUsage += HelpMessageOpt("-mempoolreplacement", strprintf(_("Enable transaction replacement in the memory pool (default: %u)"), DEFAULT_ENABLE_REPLACEMENT));
    strUsage += HelpMessageGroup(_("Block creation options:"));
    strUsage += HelpMessageOpt("-blockmaxweight=<n>", strprintf(_("Set maximum BIP141 block weight (default: %d)"), DEFAULT_BLOCK_MAX_WEIGHT));
    strUsage += HelpMessageOpt("-blockmaxsize=<n>", strprintf(_("Set maximum block size in bytes (default: %d)"), DEFAULT_BLOCK_MAX_SIZE));
    strUsage += HelpMessageOpt("-blockprioritysize=<n>", strprintf(_("Set maximum size of high-priority/low-fee transactions in bytes (default: %d)"), DEFAULT_BLOCK_PRIORITY_SIZE));
    if (showDebug)
        strUsage += HelpMessageOpt("-blockversion=<n>", "Override block version to test forking scenarios");
    strUsage += HelpMessageGroup(_("RPC server options:"));
    strUsage += HelpMessageOpt("-server", _("Accept command line and JSON-RPC commands"));
    strUsage += HelpMessageOpt("-rest", strprintf(_("Accept public REST requests (default: %u)"), DEFAULT_REST_ENABLE));
    strUsage += HelpMessageOpt("-rpcbind=<addr>", _("Bind to given address to listen for JSON-RPC connections. Use [host]:port notation for IPv6. This option can be specified multiple times (default: bind to all interfaces)"));
    strUsage += HelpMessageOpt("-rpccookiefile=<loc>", _("Location of the auth cookie (default: data dir)"));
    strUsage += HelpMessageOpt("-rpcuser=<user>", _("Username for JSON-RPC connections"));
    strUsage += HelpMessageOpt("-rpcpassword=<pw>", _("Password for JSON-RPC connections"));
    strUsage += HelpMessageOpt("-rpcauth=<userpw>", _("Username and hashed password for JSON-RPC connections. The field <userpw> comes in the format: <USERNAME>:<SALT>$<HASH>. A canonical python script is included in share/rpcuser. This option can be specified multiple times"));
    strUsage += HelpMessageOpt("-rpcport=<port>", strprintf(_("Listen for JSON-RPC connections on <port> (default: %u or testnet: %u)"), BaseParams(CBaseChainParams::MAIN).RPCPort(), BaseParams(CBaseChainParams::TESTNET).RPCPort()));
    strUsage += HelpMessageOpt("-rpcallowip=<ip>", _("Allow JSON-RPC connections from specified source. Valid for <ip> are a single IP (e.g. 1.2.3.4), a network/netmask (e.g. 1.2.3.4/255.255.255.0) or a network/CIDR (e.g. 1.2.3.4/24). This option can be specified multiple times"));
    strUsage += HelpMessageOpt("-rpcthreads=<n>", strprintf(_("Set the number of threads to service RPC calls (default: %d)"), DEFAULT_HTTP_THREADS));
    if (showDebug) {
        strUsage += HelpMessageOpt("-rpcworkqueue=<n>", strprintf("Set the depth of the work queue to service RPC calls (default: %d)", DEFAULT_HTTP_WORKQUEUE));
        strUsage += HelpMessageOpt("-rpcservertimeout=<n>", strprintf("Timeout during HTTP requests (default: %d)", DEFAULT_HTTP_SERVER_TIMEOUT));
    }
    return strUsage;
}
std::string LicenseInfo()