At least in the example on this page, it performs .fadeOut() by changing the opacity via JavaScript instead of using the CSS transition property. You can verify by inspecting the DOM
https://api.jquery.com/fadeOut/
Edit: I just realized you said "transforms". Transforms are a separate question from transitions. CSS transforms are concerned with giving an element a different size, rotation, and/or position. Transitions are concerned with changing any given CSS property gradually over time (including potentially transforms). I think you're right that jQuery started using CSS transforms, but it does not appear to use CSS transitions.
