If difference between inclination angle of plane and Slope Limit is less then 0.7, then CharacterController still can climb it.

In attached scene there is a plane tilted on 45 degree angle and First Person Controller with Slope Limit = 44.3.

Play scene and try to climp with First Person Controller on the tilted plane.
You will succeed despite the fact that Slope Limit < angle of the plane

Same goes for Step Offset.