PEP 479 might be what GP was referring to:
https://www.python.org/dev/peps/pep-0479/
PEP 479 is specifically related to generators:
> If a StopIteration is about to bubble out of a generator frame, it is replaced with RuntimeError, which causes the next() call (which invoked the generator) to fail, passing that exception out. From then on it's just like any old exception.
In fact, the Python developers recently doubled-down on the design by introducing `StopAsyncIteration`.