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

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