After process start and it's initial state, the stack is just another virtual address and the just sp another register. It can be mapped, remapped, changed to point to whatever. With overcommit even the initial state may not be entirely backed by physical pages.

Many userspace apps already do custom stack handling, it's how things like green threads work. And many non-native runtimes like .net already have custom handling for their managed stacks, as they often have different requirements and limitations to the "native" stack, and often incompatible formats and trying to isolate from possible bugs means there's less benefit to sharing the same stack with "native" code.