Many board games have information which is semi-secret, where it's possible to keep track of it, but not easy. For example, which resources you get and spend in Catan is public knowledge, but which resources you currently have is secret. If I was able to mentally count every player's income and expenditures, I'd know exactly what they were holding, but that's a lot of information to keep track of. Similarly, many games make earning victory points public, but the total number earned private.
This gives an advantage to players with better memories. It's not necessarily an unfair advantage, and there's only so much you can generally do with the information, but it is an advantage.
Given this, how appropriate or inappropriate would it be to keep a record of this information? I can write down every resource card someone collects, and scratch them off when they spend them, or keep a running total of points earned. Assuming I'm accurate, this would be even more precise than the people with good memories. But is it good sportsmanship?
Obviously, a group of friends who regularly play together can establish whatever social norms or house rules they want. I'm more interested in the general case, where you're playing with people you may not see regularly or may not know at all.