I would consider things like the hiccup parsing, defnc and the hx.hooks namespace stable. I’ve been using it personally to build punk, an application meant to be similar to REBL. So far it has served my uses with that application well.
The thing I’m not super confident in (and don’t have much documentation or examples of) is the defcomponent macro API. It is similar to Reagent’s create-class function in that it gives you direct access to the React class API, but it requires quite a bit of JS interop to use. If it’s something that people want to use often, I would give it more love and attention. The use cases you need it for that you can’t use a functional component are very few now that React Hooks are here.