I’m working on an open source notebook with built-in support for React and Typescript. It also comes with a built-in reactivity model and evaluate-as-you-type. To be open sourced soon, but preview version already available at https://www.typecell.org

I'm pretty sure I read some time ago that Starboard used Observable's open source runtime. Has that changed?

There is a starboard-observable package which uses the same open-source reactive runtime as observablehq

https://github.com/gzuidhof/starboard-observable