It hasn't been removed, but you cannot  see it in red triangle anymore (no idea why).
 
Names Default To Here(1);
dt = Open("$SAMPLE_DATA/Car Poll.jmp");
obj = dt << Partition(
	Y(:country),
	X(:sex, :marital status, :age, :type, :size),
	Split Best(2)
);
obj << K Fold Crossvalidation(5);
Also you can see it in preferences:
 

But if you enable it, it will launch every time. 
Also: Bring back the crossvalidation option in Partition platform menu 
					
				
			
			
				
	-Jarmo