private Vector3 GetPositionDelta()
{
    var current = transform.position;
    var delta = Vector3.zero;

    if (_lastPosition.HasValue)
    {
        delta = current - _lastPosition.Value;
        delta.y = 0; // new line
    }
    else
    {
        // Use the transform position as initial value, and delta will be zero
    }

    _lastPosition = current;
    return delta;
}

private Vector3 GetPositionDelta()
{
    var current = transform.position;
    var delta = Vector3.zero;

    if (_lastPosition.HasValue)
    {
        delta = current - _lastPosition.Value;
        delta.y = 0; // new line
    }
    else
    {
        // Use the transform position as initial value, and delta will be zero
    }

    _lastPosition = current;
    return delta;
}