This is the fact that the expression is ‘out of bounds’, not out of bound’. In other words, it is compulsory to add ’s’ to ‘bound’ because it is a fixed expression:I learnt that the room is out of bound to visitors. (Correct)But note that ‘bound’ will be right in some other contexts:He is bounds to go there tomorrow. The second error you make with ‘out of bound’ is a rather funny but very popular one. So, while you have something like baNd in the pronunciation of ‘band’, you should have something like bAAUNd in ‘bound’.

Source: Punch October 12, 2020 23:03 UTC