1971-S Silver Proof Ike Dollar Missing Cameo Frosting On Reverse

That is correct. intentional cameo proof coins started on purpose in 1992
Earlier coins that have it I think is incidental for the most part.

Something to look up and find out

unless someone has the answer...

As proof dies are struck repeatedly, the device frosting wears smooth and shiny. The fields also become scratched and weathered, but because they are on the die surface, they can be polished back to a mirror smoothness. I believe the progression of a die through DCAM -> CAM -> ordinary proof is one way and irreversible.

As Dearborn states, prior to a certain period or technology era, the DCAM/CAM frosting was essentially part of the die aging process. But at some point, the Mint began intentionally increasing the frosting of devices. It might have even been post-processing like light sand blasting or acid washing. I'm sure somebody here knows more about the specific process changes. I would hazard a guess around 1971, because 1970 was the last year for Kennedy 40% silver coins, and the first clad proofs are way more uniformly DCAM, much more than just the metal change would imply.

I agree. This is not an error, just late die state.