I am aware of the aperture ratio slider in the camera properties. However, I am wondering if there is a way to actually render squished in Blender, and stretch it out in the compositor later. I honestly don't fully understand anamorphic lenses, so I might be asking the wrong question.
There is a way to anamorphize the rendered image by controling the aspect ratio values
The values depend on the proportions of the container.
For a 1.33 image the anamorphic pixel ratio should be 2:1 meaning that the image should be squeezed 2 times horizontally (X axis)
The unsqueezed image would have a format of 2.35:1
For a 16X9 image the anamorphic pixel ratio should be 1.33