Possible for ABD (all but dissertation) to return?
ABD isn't usually thought of as a final status. Most people don't leave their university ABD. Most people spend a couple of years in the program, having passed quals, taken all the classes, and proposed a topic working on their dissertations. At this point, they are colloquially considered "All But Dissertation" because that describes what they've done so far not an official status. Every university will have a policy about what happens if you leave while ABD, and most will have some mechanism for coming back in reasonable time (not decades later) and finishing. Ask your advisor and graduate school or coordinator.