When the velocity movement causes a contact with a Collider2D, a slide maybe occur if the surface angle is less 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: Slide will only occur if some initial velocity is passed to the Rigidbody2D.Slide method.
Additional resources: Rigidbody2D.SlideMovement.gravitySlipAngle, Rigidbody2D.Slide and SlideResults.