PointsWithNoSlope.cs

/// <summary>
/// Check points in this list for slope
/// uses dot product for clarity
/// points must be in order to form lines
/// </summary>
/// <param name="points">points to check</param>
/// <returns>points with slope</returns>
List<Vector3> PointsWithNoSlope(List<Vector3> points)
{
    var result = new List<Vector3>();
    for (var i = 0; i < points.Count; i++) //index loop because easy to get the next item in list
    {
        //the points need to be contiguous or this won't work
        var line = points[i + 1] - points[i]; //get vector from start point to end point

        //check dot product of right vector and the line
        if (Mathf.Approximately(Vector3.Dot(Vector3.right, line), 1.0f)) //because floating point comparison
            continue;

        result.Add(points[i]);
    }

    return result;
}