My understanding is that when a feature is under development, the driver is reporting that feature as available when the user program queries it. I think there should be one of two solutions (or both of them):
a. The feature is never reported as available using the standard API. A new standard or non-standard API is used to query the list of in-progress features. This would allow special programs to be used to test new features without impacting real applications.
b. The feature is not reported as available using the standard API until a standard or non-standard API call is made to inform the driver that it is in testing and evaluation mode. In-progress features are then reported as working. This allows normal applications to be used to test the driver with the feature, with only a small change or wrapper to the application.
The table you linked to has a lot of options, and I'm not sure the complexity of having more states than working/WIP/not working in the drivers and APIs themselves is worth the benefit it would bring. That is definitely the kind of knowledge that I think should be available programmaticly, though.
But talk is cheap, and I'm no X developer, so I may have no idea what I'm talking about.