While some protocols require value attached to OP_RETURN outputs, this otherwise seems like a mistake.

Setting the value to 0 should really be done by the wallet/script creating the transaction (like createrawtransaction does for `data` outputs). However, as an additional check, similar to the high-fee check, sendrawtransaction could also check for non-zero value OP_RETURN outputs.

This should be configurable by a new RPC argument.

See also https://transactionfee.info/charts/output-opreturn-amount/

#### Useful skills:

* Bitcoin Core RPC code
* Transaction output types
* Bitcoin Core functional tests

#### Want to work on this issue?

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{
    static const CRPCCommand commands[]{
        {"rawtransactions", &sendrawtransaction},
        {"rawtransactions", &testmempoolaccept},
        {"blockchain", &getmempoolancestors},
        {"blockchain", &getmempooldescendants},
        {"blockchain", &getmempoolentry},
        {"blockchain", &gettxspendingprevout},
        {"blockchain", &getmempoolinfo},
        {"blockchain", &getrawmempool},
        {"blockchain", &savemempool},
        {"hidden", &submitpackage},
    };
    for (const auto& c : commands) {
        t.appendCommand(c.name, &c);
    }
}