I am a WordPress specialist, so I can speak to that. WordPress supports two kinds of taxonomies, and one has a hierarchy, but they are based on terms, not the taxonomies themselves.
Each term can have a single parent term. So in your example, you want John Smith to be a sub-term of authors, itself a term in this scenario.
person (taxonomy) > author (term), > John Smith (term)
Is that what you want?
Hugo supports taxonomies the way WordPress does by default, which is a “flat” taxonomy, meaning all terms have the same relationship with each other and the taxonomy.
person (taxnomy) > John Smith (term)
Yeah… your subject was accurate. A “multi-level taxonomy”, or hierarchal taxonomy. 
How would you handle setting sub-terms in TOML or JSON?