I will say (because you specifically pointed out you haven't checked - I do enjoy clear and honest communication online) that Obsidian at least deals with straight Markdown files, so while (per my current understanding) the program itself isn't OSS, the datafiles are all easily importable into other systems, and the files themselves are grep-able and mostly human readable (all text is there, but extra features and layout will not show up as nicely as in a Markdown-specific renderer) in any text editor.
I will say (because you specifically pointed out you haven't checked - I do enjoy clear and honest communication online) that Obsidian at least deals with straight Markdown files, so while (per my current understanding) the program itself isn't OSS, the datafiles are all easily importable into other systems, and the files themselves are grep-able and mostly human readable (all text is there, but extra features and layout will not show up as nicely as in a Markdown-specific renderer) in any text editor.
Have a good day X)