The sheen implementation is not yet finished. The idea is to make it glTF compliant however the BRDFs are not yet correctly implemented. That is the reason why modulating the sheen property in the above example has no effect.
The sheen properties in the documentation do all exist and I suppose they will not change anymore since we recently agreed on a respective naming convention.
I suggest you keep an eye on the following issue at GitHub which tracks the sheen BRDF implementation:
then maybe I don´t understand - but, lets say we only have roughness, metallness, diffuse, and normal-maps + no envMap + one directional light + applying sheen @Mugen87 you say, I should be able to see sheen applied?
sheenColor: 0xffffff, sheenRoughness:0.5 → I do not see a single “yellow” shimmering on the model´s surface at all?