When you click an "invited to review proof" notification link in the email, it automatically launches a new browser tab and loads the new proof. You cannot however quickly toggle to previous proofs from this view. You have to go back to the associated smartsheet to and reload the proof from there to be able to see all proofs. It would be great if you could toggle directly from the screen as I often want to go back and reference the previous proof to see if the changes were properly made.
Oddly though, I noticed when someone "comments" on the same proof and tags you. When you click the link in that email notification, it loads the proof in a new browser tab that you can toggle through the various proofs. It seems to me if it works when someone comments on a proof, then it should also work with the proof notification itself.
See attached screenshots!