When the gravity movement causes a contact with a Collider2D, slippage maybe occur if the surface angle is greater than this angle.
Use this angle threshold to control whether slippage will occur on surface slopes above the threshold.
The angle is in degrees and is relative to the Rigidbody2D.SlideMovement.surfaceUp vector.
NOTE: Slippage will only occur if no initial velocity is passed to the Rigidbody2D.Slide method.
Additional resources: Rigidbody2D.SlideMovement.surfaceSlideAngle, Rigidbody2D.Slide and SlideResults.