Our use case is: we have an organization that would sign in to only use the REST API with a web app we have built.
The users do not need to sign in to view the dashboards, so having a single shared user to just present the data within our application is ideal.
For example: we have an application that uses the Splunk REST API to gather some data and display that data within the larger application. We want to simply have a user use our application, but not worry about authorization with the Splunk server.
i think you can create a splunk service user account (a normal user account with least privileges, enable privileges it to run REST API's)