Possession has been losing popularity recently, up to the point that most people are trying to avoid this card or make a deal in chat to not use it. Because the configurable banned-cards-list isn't implemented yet, this is a temporary solution that just always puts Possession on the banned list.
This means it won't show up at random in your kingdoms anymore. You can still add it to the required cards if you want to play with it. It's also still possible you find a Possession in your Black Market (but only if the kingdom already has potions).