Verity returns all IAB hierarchy tiers for both versions 1 & 2 of the taxonomy:
IAB V1 – 2 tiers - 372 categories
IAB V2 – 4 tiers - 698 categories
For example, Verity analysis of an article on “The Rise of Alternative Venture Capital” identifies IAB v1 categories in 2 tiers, and IAB v2 categories in 4 tiers.