When I was an undergrad (UC Berkeley) all Math and CS classes beyond basics (like Calculus etc) allowed "cheat sheets" in exams and some were "open book" (i.e. bring whatever the fuck you want (nothing digital of course, just paper, no calculator)). I don't remember an incident when bringing a cheat sheet was useful during the exam, maybe 1 time for a minor thing. Writing out the cheat sheet before the exam definitely helps learning though. If it's something you can look up during the exam, it's not gonna be something professor will ask you. If it's something you need to read, understand and conceptualize during the exam, it's gonna take you long enough that it'll be a significant disadvantage.

EDIT: Now I remembered haha, our first CS class even printed out standardized cheat sheets and gave it to everyone. They basically printed ALL the slides of the class in tiny font on 3 papers and handed out to everyone with the exam. Fun times. I didn't even open the cheat sheet lol.

I remember I was taking Algebra 2, which was about Module Theory and Galois Theory. We asked prof if we can bring cheat sheet to the exam, he was like "I mean I don't care, you can even bring the textbook". So I did but, obviously didn't even open it. If you really need to remember something (e.g. definition of Noeterian Ring) professor may write it in the question anyway, or if you need to remember something basic (e.g. definition of Group) you could technically use the textbook, but at that point, you'll likely fail the exam anyway.