Spoilers are fair game within the pages of an encounter set.
Core Set encounter cards are used in all campaigns, i can not guarantee that there are no spoilers if you have not played everything released.
If you are afraid of spoilers, avoid pages for sets that are from campaigns you didn’t finish yet. Cards from the Core box are used throughout all campaigns, so they are a special case. For them, I will try to keep spoilers from specific campaigns to a minimum, but it won’t be completely avoidable.