Oh, yeah. But I think It is probably true that it is difficult to build a model of a continuous trait in which truncation of one tail does not affect the equilibrium of the other tail.

The more relevant point is additive heritability (aka h^2 or narrow sense heritability. Any model will have some, so my condition of having any is not helpful. But if a trait has a lot, that means the trait is pretty close to counting genes, hence the distribution must be a bell curve. But that doesn't mean that it is a constraint on models.