From what I remember his example was a reserved field which you must set to 0 when writing, and ignore while reading to enable extensibility. In a modern conte

From what I remember his example was a reserved field which you must set to 0 when writing, and ignore while reading to enable extensibility. In a modern context this would correspond to ignoring unknown keys in json, or ignoring unknown fields in protobuf. Sadly many interpret this as accepting an ill defined superset of the specification, which then turns into a complex specification as implementations rely on those extensions being handled in a specific way.