What are you using to replace etcd here? Where is state stored?

They implemented a store in TypeScript: https://github.com/ngrok/webernetes/blob/main/src/cluster/st...

etcd is not a requirement of k8s, plenty of clusters out there use non-etcd stores. E.g. k3s/k3d can use sqlite, all of the big cloud providers use their own in-house datastores instead of etcd.