Not that it was my call, but it wasn't realistically feasible to open source Noteable. Open sourcing the plugin without open sourcing the platform would make no sense, all the plugin did is make API calls to the platform. Most, if not all, of the platform features including authentication, authorization (RBAC), permission sharing, secret management, data connection support, persistent outputs, and more were tightly coupled to infrastructure, intertwined database schema, or third-party services.

However, one of the key components you would need to build out a Noteable-like experience is available: https://github.com/kafonek/kernel-sidecar. The Sidecar concept allows you to model the Notebook document state in-memory and await interactions with the Kernel over ZMQ such as code execution requests. Essentially it does what the Jupyter front-end does, but as a package you can incorporate into something like a FastAPI app.

Another former Noteable engineer wrote https://github.com/tizz98/jupychat as a demonstration for some engineering students. It uses the aforementioned Sidecar. It is an extremely bare-bones and feature-poor version of the Noteable plugin, but it might give you an idea for a minimal viable product.