While work on REST API has started, the development of the project has since taken an extended period because of the aim the project has nowadays—growing exponentially. In the beginning, the REST API was just a method of underlying data in WordPress, and some of the code found in the first version still exists nowadays. The limit is set at four core objects such as the users, posts, taxonomies and metadata that were decided early on. Even if this seems somewhat set on limitations, it will set the REST API to cover options and any other kind of data types. The REST API, being identical to the core of WordPress, was built with extensibility in mind and thus includes the infrastructure for handling the remaining data types in doing anything that you'd like as a developer.
The infrastructure of the API will support nearly everything, and if the core endpoints are taken away, it will be considered as a framework for building APIs that can be built any way you'd like. It,...