Are you referring to the Candidate Information Bulletin that is linked in the ATT notice email? If so, I've read it before and the sheet protector question is unclear in the text- which is why I posted the question here to find out others' experience. Within that bulletin, under "Reference Materials" section, it is stated that "...reference materials must be bound and remain bound during the exam. Bound refers to material permanently bound by stitching or glue and materials fastened securely in its cover by fasteners, which penetrate all papers." To me there is no definite answer to my question in this text, and I'd expect the Prometric proctor to feel similarly. Therefore I am suggesting to err on the side of caution by bringing the puncher unless the Board updates the bulletin to shed light on the issue.
PS: Thanks for your lightning-fast responses!