(example: 2.79b release)
(example: 2.80, edbf15d3c044, blender2.8, 2018-11-28, as found on the splash screen)
Short description of error
When Unwrapping an object on a rectangular image, the UV respects the aspect ratio of the image and stretches in the longer dimension
The "Correct Aspect" option lets you choose if you want to neglect the image ratio or not, That was the case in 2.7X, now when you toggle this option nothing happens!
as I said, it only works if the image selected in the uv editor isn't a perfect square.
Exact steps for others to reproduce the error
create a cylinder for instance, select a vertical edge of it, and mark it seem, go to uv editor, and make an image that isn't square, select all, hit U, select uv unwrap, now toggle the " correct aspect" on and off, nothing happens.