This was a good idea based on suggestions from many of us here in the forum. Unfortunately, in practice, the feature is kind of “half-baked.” It often incorrectly reports required rules as being unused, and it fails to correctly identify unused rules that can be deleted. Nevertheless, it does provide a starting point.
The problem is, the unused CSS appears in a fixed modal, and blurs out the background of the website. If you preview the website first, and you have your Styles palette detached and on another monitor, (or use an external editor) you can experiment with disabling classes and see if they affect the website. But there is no way to export a text file of the unused CSS classes, nor can one one simply copy the unused classes by highlighting them with the mouse. This seriously limits the usefulness of this dialogue. Basically, if you’re limited to one monitor, you have to make notes, close the dialogue, and then go through the process of elimination.
This feature would be much more useful if there was an option to export the unused CSS as a text file, or allow us to copy the classes from the modal dialogue, or (ideally) not have this unused classes appear in a modal at all, but rather in a regular window that can be moved around, and which doesn’t disable or blur out the website beneath it.