Cine camera doesn't expose the bConstrainAspectRatio toggle in the details panel

1. Drag in a cine camera
2. Look in the details panel for a toggle to "Constrain Aspect Ratio".

It doesn't exist because it's hidden by default for cine cameras. In fact, cine cameras hide all properties in the "Camera Settings" group. If you drag in a regular camera, you'll see the "Constraint Aspect Ratio" property.