If you see this error, it means that the message you are trying to publish contains some arbitrary field(s) in the extras payload that is not allowed by the Ably service.
The extras payload in the message is currently reserved for some specific objects related to the Ably service, however, we are aiming to add a custom object within the extras payload so developers can use this to add their custom data that's not supposed to be part of the actual message payload. You can track the progress on this via the GitHub issue. We'll update this article if/when that's implemented.
For now, to solve this error, please include the object you have in the extras payload as part of the message payload itself. If you have further questions, please feel free to reach out to us and we'll be happy to help you out.