The Canvas feature is a custom file format, but they published the format here: https://github.com/obsidianmd/obsidian-api/blob/master/canva...
If Obsidian goes under or you want to migrate at least your data isn't lost.
The challenge with Canvas is that there is no standard format for this type of data. As @milch mentioned Obsidian's .canvas format is simple JSON, that is stored locally. Like .md files, they are individual .canvas files in folders on your device. It remains to be seen whether .canvas can stand the test of time, but at least the data is in a format that will make it easy to convert to and from, and is relatively human-readable given the constraints of having to store positional data. There are efforts to grow the adoption .canvas, but like Markdown it will take time for that ecosystem to develop.
That’s true, but with the canvas feature they broke the promise of your notes just being directories of markdown files and attachments. I really, really like Obsidian for its core functionality and I worry that they are going to keep adding features and end up ruining it.
For me, the best news would be them announcing it is feature complete and switch the project into maintenance mode. Bug fixes, performance improvements, and platform updates.