RainbowCake is production ready! It has been in production for nearly two years now, in several applications, in the hands of (at least) tens of thousands of users.
The development of RainbowCake started within AutSoft, where the framework is being used to build greenfield Android/Kotlin projects (such as Sender!), and it also serves as the foundation for applications that are being refactored from Java to Kotlin (such as a certain HR application). Both of these examples are running on RainbowCake in production.
There are also multiple non-public projects using RainbowCake, for example, the 150-screen, 100kloc app discussed in this talk.
You can take a look at the code and participate in the project on GitHub.