I know the defensive posture for stabbing thrusts. But what about slashing
Attacks. I often wondered what you would do, if attacked. I imaging the arm forces
Must teach a technique to defend this type of attack.

Karate class would too I would think

There are several ways, basic akido teaches the basic methods for this, the military and some police academies teach a modified form of this. It will still be a step away, and a arm grasp along with using the motion of their movement with them. So the angle of the movement of the knife will normally determine which block and action is done.

Also it will depend on the positoin the knife is held, blade forward or reversed ( advanced use of blade)