From what I know the GIL is released by calling PyEval_ReleaseThread() or PyEval_SaveThread(). 

I see we can use PyThreadState_Delete() to clean up-specifically destroy thread state but does not release the GIL and requires a thread state. On the other hand, PyThreadState_DeleteCurrent() allows to both clean up the current thread state - no thread state is required as well release the GIL which is convenient. I would not fight so much to keep it public given we have PyEval_ReleaseThread() or PyEval_SaveThread().