They were always percent values, it is just that they used to be expressed in pixels with no reference point. There was (and still is) a setting for the thumbnail size.
You can set the thumbnail size to the largest dimension and then set the cropping percentages to get the aspect ratio you want.
For instance, a 100 pixel thumbnail with 0% left and 25% top cropping will give a 2 to 1 aspect or 100x50px thumbnail.