I think it matters to what extend the representation in the image is derived from reality (as opposed to creative decisions made while modifying the image), really. Stacked exposures and photo stacking are largely there to remove the effects of the camera on the image, and better reflect the underlying reality, so I would still call the result a photograph. (color grading and such are often far more driven by creative decisions than the underlying reality and still that doesn't usually disqualify a photograph. In-camera visual effects are also still photographs).