A comment history would be very helpful for the designer. A client might accidentally delete feedback and for the designer to have the capability to view any and all feedback (resolved/ deleted) would be helpful. That way it can be easily retrieved and the client will not have to worry about having to make those comments all over again. It would be a good safety, "backup" feature.
Thanks for your feedback. You can already view all of the comments that have been resolved and not on the prototype, but not the deleted. Just to be clear – are you looking at a timeline for comment activity? Or is it just to be able to see deleted comments?