Honored to meet you, Johnny!
I recently learned about the Johnny Decimal System, while doing some "research" following links on Zettelkasten and other systems.
I found your system brilliant, how simple you kept it, with consistent, easy-to-remember rules. I like that it has no dependencies on service/platform or language, it's generic in the best sense of the term.
As I was reading, I immediately started creating a folder structure, and took notes (in area "00 System" :).
Interestingly, I felt that the system builds on how people already keep folders and notes in ad-hoc categories - and giving it a few organizational concepts to standardize the structure. Like, I have a big nested folder of notes spanning years, and migrating them to a newly started decimal system, it's giving all notes a canonical place, with a defined schema/model/taxonomy.
As I wrote in the parent comment, I have a few aliases for shell commands, like find by file/folder name, grep to search notes, a shortcut to add a note. I'm now thinking of adapting them to the decimal system, so I can, for example, list all categories in an area, or add note to a category. I live in the terminal (and code editor), so having such shortcuts, usually of 1~3 characters, makes note-taking a natural, effortless step. I like that you have a page for such workflows from the terminal:
https://johnnydecimal.com/concepts/working-at-the-terminal/
For feedback, one thing comes to mind that could help beginners like me, would be examples on the site, like downloadable template(s) of common project organization. I had difficulty coming up with a sensible structure of categories - in fact, the "root" project where I'm moving my existing notes to, the foundation is still not stabilized, deciding where everything should go. It might help if there were some examples or ideas of categories, that could be copied and adapted.