"Stackless" is a large topic with a convoluted history. Do the web search. In short, no, it will never go in the core - too disruptive to too many things. Parts have lived on in other ways, watered down versions. The PyPy project captured most of what remains, as optional features. CPython's generators captured the easiest part: after a yield, the C stack space the generator consumed is released, while the Python VM stack space lives on in the heap awaiting possible resumption (but, if/when it's resumed, C stack space is required again).