Cluster stats flat settings

I'm trying to get cluster stats using the flat_settings query parameter. However when I run:

GET _cluster/stats?flat_settings=true

I get back the following response:

{
  "error": {
    "root_cause": [
      {
        "type": "illegal_argument_exception",
        "reason": "request [/_cluster/stats] contains unrecognized parameter: [flat_settings]"
      }
    ],
    "type": "illegal_argument_exception",
    "reason": "request [/_cluster/stats] contains unrecognized parameter: [flat_settings]"
  },
  "status": 400
}

What am I doing wrong here, is this a bug or user error?