PEP 523 was to give user code the ability to change the eval function. While the work was motivated by our JIT work, supporting debugging was another motivating factor: https://www.python.org/dev/peps/pep-0523/#debugging. There's no C API because at the time there was no need as PyInterpreterState was publicly exposed.

I don't think anyone is suggesting to add something to the stable ABI, so this comes down to whether this should be exposed as part of the CPython API or the publicly accessible internal API. Since there is no distinction of "you probably don't want to use this but we won't yank it out from underneath" you I think this belongs in the CPython API somehow. Otherwise someone should propose withdrawing PEP 523 as I think that shifts what the PEP was aiming for. You can also ask on python-dev or the steering council (which I will abstain myself from due to being a co-author of PEP 523).