Disclosure: I work on Tree Notation. It’s the future of file formats, IMO. The idea is to have 2 levels: a simple, minimal syntax/notation (think binary) called Tree Notation, and then have higher level grammars on top of that, called tree languages. It works for encoding data and also for programming languages, regardless of paradigm. https://github.com/treenotation/jtree
From a quick glance, my issue with Tree Notation would be that it's not enough syntax, i.e. it does not provide enough structure for me to grasp the overall structure with a cursory glance. Maybe it would work better if GitHub had a syntax highlighting for it. (But requiring syntax highlighting to be readable is a large red flag on its own.) Or that's just a feeling that would be mitigated if I saw larger files.
Your impression is correct though, without highlighting it’s awful. Try the Tree Language designer app to see it with highlighting, type checking, autocomplete etc...there are interesting ways to accomplish everything without syntax, often not obvious. But tooling is essential to make it better than existing options. Help wanted!