The old 'chai2-json-schema' dependency had a banUnknown property and used it internally as an argument to tv4.validateResult(). When #55 was implemented, the property was just transitioned to tv4 module export. Unfortunately, that also rendered it useless for its intended purpose. Our unit tests still passed as none were really testing this option. As such, the code has essentially been using banUnknown = false for several years now. While its intent could be trivially added to the existing validation routine, I'm unconvinced we should. There seem to be two schools of thought on the capability, without consensus. Per ajv-validator/ajv#238, the ajv developer doesn't intend to support it.