You've hit on an important point here. As soon as Apple documents one of their CPU extensions, they've put themselves in the position of having to support it lo

You've hit on an important point here. As soon as Apple documents one of their CPU extensions, they've put themselves in the position of having to support it long-term. Leaving it undocumented gives them the freedom to make breaking changes from one CPU generation to the next -- hopefully with the end goal of deciding on an implementation and documenting the end result.