if (platformRB2d.velocity.y <= 0)
    {
        gameObject.GetComponent<BoxCollider2D>().enabled = false;
    }
}

private void OnCollisionExit2D(Collision2D collider)
{
    if (collider.gameObject.tag == "platformManager")
    {
        Debug.Log("!!!!!!!!!");
        gameObject.GetComponent<BoxCollider2D>().enabled = true;
    }
}