WordPress has had custom taxonomies for years, but there is no way to manage them. While built-in tags and categories can be converted into each other, they cannot be converted into custom taxonomies or vice-versa.
The result is functional uselessness of custom taxonomies for any site which has already made extensive use of tags and categories.
Further descriptions of this problem:
Currently open support request looking for a solution to this problem:
After searching for weeks, it is clear that there is no solution at the moment. An opportunity for a plugin developer looking for a new project? Or a feature to add to any of the existing tag/category/taxonomy plugins?
It is also seems like a glaring need within WordPress. I don't understand why custom taxonomies are in the core, but the ability to make use of them is not.