1) You are always putting yourself at a disadvantage if not using updated reference materials.
2) For the CPB, it is only a small disadvantage. Per the AAPC website, of the 200 questions, only 15 are coding questions. The odds of those 15 being specifically related to something that changed between 2020 and 2021 are rather slim.
3) How would you feel if you did fail by 1 question and it was coding related?
From a practical standpoint, if this was a coding certification exam, I would strongly encourage 2021 books. However, it is not a coding certification exam. In your situation, I would say your 2020 books are fine. I would make notes in the 2020 books regarding 2021 changes. Specifically for outpatient E/M which changed a lot.