By no means an expert here, but it seems to me reaching out to your POI (or whomever you have had the most contact with) would be a good idea to reaffirm your interest. My guess is that in the event their offer is turned down by one student, they probably want to avoid having another offer turned down. The professor might also be able to shed some light on the length of the waitlist/the odds you’ll get off of it. I think this is a reasonable thing to ask about—especially if you have an offer elsewhere that you’re considering, though I wouldn’t necessarily say you have another offer..In my case, I didn’t have to ask about an indication of the length/if they think someone might not accept an offer because my POI kindly sent me an email explaining the situation, so I took the opportunity in my reply to reiterate that I would like to stay on the waitlist because I thought their department was a strong fit for me. Hope this helps!