The Observable runtime is open source, and trivial to integrate into your existing react/whatever webapp.
There is also a vscode plugin that allows for live editing and rendering. It doesn't give you the ObservableHQ interface, but tbh I like working with files more. It also allows me to git everything.
https://marketplace.visualstudio.com/items?itemName=GordonSm...
You can't self-host the same way you can self-host the Jupyter Notebook or Jupyter Lab environment.
Or has that changed? Can I work on something offline without communicating with their servers?
You can self host observble, but you can't self host observablehq.
Literate programming just means the combination of code and documentation/visualisation.
Observablehq provides a nice inline code editor, but that's incidental to what observable is.
Git can also be used with a GUI but that GUI is not essential to what git is.
Simularly observable notebooks can just as well be created without the GUI, in a quite convenient way.
Knuths original literate programming environment consisted of a command line tool to split files that mixed pascal and tex into the pascal and tex source, to be compiled separately.
Which means that according to your definition, the original literate programming system, isn't literate programming.
All the best.
Hope you'll look at it again in the future. In a sense it's your loss, so good luck and take care.
It's like saying git (observable) can't be self hosted because github (observablehq) is not open source.
But observable (the reactive notebook technology) can do everything a jupyter notebook can do, you just need to write a bit of communication code to hook up a websocket. (For me sandboxing the ui is a feature not a bug.)
And you can also self host it on a machine of your choice, and edit it locally.
We do that internally for all our dashboards.