Slow a rigidbody to a stop over a set distance?

You can just use the physics equation to calculate deceleration.
Edit: Changed the code so that if the position is not equal to the desired position and it skips the position it still works. For eg. if you want to decelerate at y= -1 and the body goes from -0.8 to -1.2 the code will still work.

 private bool firstTime = true;  
 private Rigidbody rb = GetComponent<Rigidbody>();

Write this code in fixed update

 if(transform.position.y<=yPos)
 { 
     if(firstTime)
     {
          firstTIme = false;
         float deceleration = (((rb.velocity * rb.velocity) * -1)/distance)
         rb.velocity=deceleration;
      }
  }