The REST API documentation published here on the Magento 2 Developer Documentation website describes the REST APIs that are available on the latest release of Magento 2.

This documentation is generated from the code base using Swagger and represents the state of the code at the time the documentation was generated. Swagger displays a list of service names. Click on a service name to display the list of APIs defined within that service. Click on an API to display detailed information about that API.

You can also create a dynamic REST API documentation set on your server with live data. On your server, Swagger displays REST APIs for all installed products and allows you to try out the APIs. For more information, see Generate local REST API reference.

The REST API documentation on the Magento Developers website (devdocs.magento.com) is static. It is not running an instance of Magento, and therefore it does not have live data.