Costus root oil has one of the more divisive profiles in perfumery. It is usually described as animalic: warm, dirty, and specifically like unwashed hair or wet dog, with a violet-like sweetness underneath. Used at trace levels it gives a skin-like depth that is difficult to obtain any other way, and it was a component of several classic twentieth century compositions.
It is now essentially banned. The International Fragrance Association restricted costus root oil after it was identified as a potent skin sensitiser, and the material was withdrawn from general perfumery use. Reformulations of older perfumes that relied on it are among the more audible casualties of the sensitiser restrictions, and substitutes have never quite matched it.
The restriction applies to the oil rather than to the root, which is why costus continues in medicinal and incense use in the traditions that use it while having largely disappeared from Western fine fragrance.
That split is worth noticing as a general pattern. Several traditional aromatics have been regulated out of one market on toxicological grounds while continuing without interruption in another, and the difference is usually the route of exposure: a burned resin, a snuffed powder and a skin-applied oil are three different risk profiles for one substance, and only the third is what the fragrance regulations are about.