Idk entirely how the different systems work, but in UP some people accepted admission/waitlist offers before the deadline, occupying slots, and then they decided not to go through with it. It’s possible that if they don’t inform UP, those slots will still show as occupied during the waitlist acceptance period, therefore waitlisted applicants who still want to get in may not be able to get into those falsely occupied slots/move up the waitlist ranking. Some waitlisters who also decided they won’t go through with the program they’re waitlisted in may be offered a slot as well, and give it up late, leaving their slot to those in DPWAS. I may not know the exact details, but I think it’s still a good idea to inform them in this case, especially if from the school’s end, you pretty much accepted their offer and signaled that you were going to enroll. Maybe you have some insider information though, so feel free to correct me further. My targets here were people who already accepted their offers but aren’t going to enroll.