…the ability to create and use custom taxonomies has been around since 2007. We didn’t get all the cool functions added until 2.8 though.
However, one thing we’ve had since 2.3 was the ability to create taxonomies for any object type, not just posts. In WordPress, there are several object types:
So, you can technically create a taxonomy for any object type. Most of WordPress core support is for posts, but the API is extremely well thought out and can handle the other object types with minimal code effort.
This tutorial will focus on registering and using a taxonomy on the user object type. It will not be a 100% solution for everything you can do with a custom user taxonomy. Consider this an extremely rough draft of what’s possible.