I think they wanted to avoid having people build on it before they stabilize its interfaces, since they still had some big changes planned. As I understand it, the plan is supposedly still to publish it at some point.
I hope that if that's true, they publish it soon, because its design is very relevant to ongoing changes in the ecosystem, and it could be a useful reference point. :(
[1] https://github.com/Shopify/runix/