The data is open and stored in markdown format. Plugins are open source. The core product is not open source, but it's also just an electron app. I've always viewed Obsidian as the inverse of an open core product.
The licence method we went with for AS Notes (https://www.asnotes.io - a wikilinks and markdown based notes / docs / blog extension for VS code) was to make the client (extension) fully opensource with a public / private cryptographic licence key model, with a couple of pro gated features. I think an opensource product is very important for a notes product, where the implications of loosing access to a tool are huge for users that invest a lot of time in a knowledge base.
> I've always viewed Obsidian as the inverse of an open core product.
I'd like to hereby propose the "open shell" development model.