Was it considered cheating that the English goalie had statistics about penalty kick preferences of opposing players written on his water bottle (and concealed under a towel) in the semi-finals for the Euros?
no - the key difference is those penalty kick stats are derived from watching the games, which are public, not from surreptitious surveillance of private practice sessions
If I have a penalty kick and you’re the keeper, I want to know if you know where I kick the ball most often. If I know that you know, I will kick somewhere else.
So if you’re the keeper and you know my habits, you have a slight advantage if I don’t know that you know. So you would hide the list.
I can't say for them, but towel is good heat insulator and can keep bottle content cold. I use it often for that reason (either towel or other clothes that's already in backpack).