> general purpose data structure format

General purpose until you need to store dates/times, self-referential structures, enums, etc.

json and toml are both serializations of data. I don't think the line you're trying to draw between "configuration file format" and "data structure format" is a well-defined line.

Sure it's a blurry line but some formats are better than others for some specific tasks. JSON isn't the right tool for every job but it's a good enough lowest common denominator for data sharing.

I wouldn't want to use if for config files or passing complex data but then it's not designed for that.

TOML also has more "normal" notations for dicts and lists that are almost the same as JSON.

There is a distinction between markup languages e object notation languages. XML, HTML and Markdown are markup, JSON, YAML, TOML and most other are object notations.

(here my distinction, in general, is if they allow unquoted text)