Where is Storage and LocalStorage in rc0?

As @gasaki already mentioned you should use @ionic/storage.
This uses LocalForage when running in a desktop browser only if no “better” option is available or e.g. SQLite when running on a device.
Here is the description:

/**

  • Storage is an easy way to store key/value pairs and other complicated
  • data in a way that uses a variety of storage engines underneath. Currently,
  • Storage uses localforage underneath to abstract away the various storage
  • engines while still providing a simple API.
  • When running natively, Storage will prioritize using SQLite, as it’s one of
  • the most stable and widely used file-based databases, and avoids some of the
  • pitfalls of things like localstorage that the OS can decide to clear out in
  • low disk-space situations.
  • When running in the web or as a Progressive Web App, Storage will attempt to use
  • IndexedDB, WebSQL, and localstorage, in that order.
    */