We've just discussed this in the last couple weeks. We are planning to add a created_at field as well as a last_updated_by field to most (all?) API resources. Certainly all the revisable resources would be included, we're evaluating if there's value in having it on all fields.
Try AEC Bookmarks chrome extension that displays the author of the last modification (hover mouse cursor over the last modified date on the rules/ data elements/ extension pages).
