Unfortunately, it’s there, but it’s hidden from the details panel. I don’t know of any workarounds that don’t involve modifying code. If you can modify code, you can just remove this segment from CineCameraComponent:
HideCategories = (CameraSettings)
Cine camera derive from regular cameras. The property is on the regular camera but the cine camera hides all the properties in the “Camera Settings” group in favor of the “Current Camera Settings” group. Unfortunately this “Constrain Aspect Ratio” toggle is in the “Camera Settings” group.
This should be fixed in code so that it only hides the irrelevant settings, like aspect ratio, which for the cine cameras is a value that is determined by the filmback settings. I’ve filed a bug and you can track it here once it comes online.