I found some reference on the russian unity3d site. The code shown there says "yes, it does exactly what you said, except a) the two parameters are reversed, b) the "planeNormal" (which can be any vector) doesn't have to be normalized."
Where did you find the info that there is a function called Vector3.Exclude() in the first place?
Here in the future-times of v4.6, Exclude is deprecated in favour of (the now documented) Vector3.ProjectOnPlane - Note that the order of the parameters is swapped.