Mystery of Intel Cpuid 21H Leaf

The latest June 2021 SDM manual volume 2A, chapter 3 added a mysterious note about CPUID leaf 21H:

"Invalid. No existing or future CPU will return processor identification or feature information if the initial EAX value is 21H."

Any idea why this is specified? They claim the leaf will just always return zero, or if max leaf is lower, returning the highest valid values as usual. This makes it forever just unused/unusable. But why?