Are you using linked css design areas? If the css file is live/public and has static cache turned on you should be seeing it served out on __data paths, but only if you use linked css.
I'll pass this onto our documentation people. The functionality is still there, I have it working on the latest stable Matrix, in fact we use it quite heavily so the browser caches the CSS properly and Matrix isn't used to process CSS for each page load (passed off to apache to serve).