Using v0.30.2, Hugo does indeed create a “categories” and “tags” taxonomy if you don’t specify anything in your config.
This is the output with a plain config.toml with no taxonomies defined:
0 tags created
0 categories created
If you do specify taxonomies in your config, it will only create what you choose. So, if you were to only define tags in config.toml, it would no longer output the “0 categories created”, because categories don’t exist anymore.