Hi! 5 card mandatory cash in has been in a rule since Risk's creation and is an element in official rulebooks in almost every variation of the game. The strategies of saving up for an opportune moment and depriving other players of cards are in fact facilitated by this limit - you have to be careful, and time your attacks so that you can take out an opponent before they reach the card limit but when they have a lot of cards.
Kyle Owens