usched is a light-weight cooperative-multitasking-coroutine library that I wrote (github.com). To switch between threads, it copies the stack of the outgoing thread out to somewhere (e.g., the heap) and copies in the stack of the incoming thread, that was similarly copied out on the previous context switch.
As usual, whatever you invent, eventually turns out to be known since long time ago. In this case, we are talking about something that happened at fabulous times, almost at the dawn of history, as far as computing technology is concerned: Algol-68. Dr. J.J.F.M. Schlichting's PhD. thesis Een Vroege Implementatie Van Herzien Algol 68 (An Early Implementation Of Revised Algol 68) describes in detail an implementation of that language by Control Data B. Y. and the University of Amsterdam (among others).
On p. 4-30 we find:
The term thread is not used (maybe not yet invented, definitely not current), but we see a sophisticated compiler-assisted cooperative multitasking with fork-join concurrency and cactus stacks:
