On the first call it was stated that the specification should not contain optional parts. A “common LUT format” where implementations are not required to implement support for all parts of the format loses its commonality.
I can only find one instance where something is specifically labeled as an extension and is not required. Appendix D, bullet point #1 says, "
IndexMaps with dim > 2 are an extension and are not required".
Are there other features that are specifically called out as being optional?