It wouldn't have been draw.io / https://app.diagrams.net/, would it? I use that tool frequently, and loved that feature at a company where data couldn't be stored on external servers

in the case of Kinopio, you can also export your data to a JSON file. But I think the OP may be referring to something like obsidian where each document corresponds is an individual markdown file which is always up to date

It's a hidden, opaque file. As far as I can tell, my browser doesn't tell me what its path is on my file system. That path and the file format could change with a browser update. Again, localStorage is nice, but this whole setup isn't comparable to a .pdf or whatever that I can have on my desktop.