I believe by definition an extraction/extracted image is a 3rd party edit of the image to remove the background and other characters, so an artist really can't release an extraction. It's also particularly not likely to happen, as most likely the original artist could just simply turn off the background layer in the original image.
Extracted images don't seem uploaded too frequently (or at least labelled properly), but of the more recent ones there has been a few recently approved. I think like any other type of photoshopping it is a judgement call on whether or not the editing detracts from the image or adds to the image in a way considered good enough to outweigh the nature of it being a 3rd party edit.