The SBC-4 standard will continue to collect new features as they are invented. Each feature set picks a subset of the full standard. Devices can also implement features not defined in their advertised feature sets.
Features can be added to a feature set over time; a version number will be incremented when this is done. That avoids breaking older software, which may rely on anything in the version of the feature set to which it was designed.
Removal of a feature requires defining a new feature set, since that could break older software. If some command becomes uninteresting, devices would implement it as a NOP to maintain software compatibility rather than return errors.
It's not perfect, but we hope this will be simpler for software to manage than worrying about every individual bit and field.