The Burroughs mainframes stored most of their stacks in main memory - they have a couple of deep stack addressing modes (and a proper set of display registers).

Alternately 29Ks and Sparcs have register windows which are essentially top of stack caches.

I think that the RTXs were a thing at a particular point of time that made sense before caches moved on-chip, they didn't catch because they expected you to buy into the forth world-view (rather than providing C and other languages of the day)