is a good example of an actual database that uses plaintext files in your filesystem.
I can see the argument that doing this with JSON is better (or worse), but regardless, Recutils is an interesting idea that i wish more people knew about. I can imagine a lot of cool things emerging if people would iterate on the idea.
Anything that stores data on a computer is essentially a database. It's all about representation and what kinds of operations you prioritize for performance.
Apache Spark / Databricks is an example of this. Parquet files are stored in folders. A folder is assumed to hold one dataset split into multiple files based on specified partition criteria. The VMs read the necessary files into memory and then operate on it.
