I am getting lines in a Smooth Shaded cone, I don’t remember having this problem before.
There are no duplicate vertices, I have supporting geometry, face normals are the facing the right way, Edge Split modifier doesn’t change anything, and this also happens if I just add a standard Cone Primitive.
Did you look if your model have any custom geometry data? It’s common that models you download from internet have this kind of problem due to this custom geometry data. If you delete all that the problem disappears.
Anyway, I cannot be sure that that is the problem without seeing the file.
Yes, normally those techniques work just fine, but it seems that cones are shapes not well handled if they are low poly.
I believe that the biggest problem is that the faces are too stretched, not even close to squares, what is not really a big problem in cylinders because they don’t get thinner on one side besides the stretched faces.
If you make loop cuts to approximate the faces to squares it becomes much better, but it will not be low poly anymore.