With the introduction of ChirpStack v4, the REST API interface is no longer included. Historically it was included to serve the web-interface, as at that time, gRPC could not be used within the browser. The included REST interface internally translated REST requests into gRPC and back.
It is recommended to use the gRPC if possible. However if a REST interface is required, please refer to https://github.com/chirpstack/chirpstack-rest-api. This component provides the same translation layer (using the gRPC Gateway project), including a web-based interface of the API (using Swagger UI).