Plugtest 2021: Specification discussion: non existent attribute in a json body should be ignored

Used as a placeholder for the specific issue below:

When receiving a json body and an attribute is not defined, the current expectation from the plugtest is to receive a code 400. Is a more appropriate action to ignore the unknown parameter? This allows a less strict approach. It was not found in the spec what should be the behavior as a result of this situation. Need a clearer spec indication in order to standardized the implementations.