It's more "nobody else is interested" than "it's not out in the open", but I've made my own structured data format implemented as a Rust serializer https://github.com/rsaarelm/idm and am using it for a growing collection of command-line tools for managing personal notes written as outline files https://github.com/rsaarelm/idm-tools and to run a static site generator https://github.com/rsaarelm/blog-engine . I'm also writing a game that uses IDM as the data serialization format.

Idea for the format was that you can write structured data with a really minimal syntax if you have an external type schema running the parsing, and the syntax emerged from the line-and-indentation based outline note files I'd started writing for myself. It took some months of work and planning and a couple rewrites to get the core IDM library working right. The tools and site generator were simple and straightforward in comparison.