OpenSearch/rest-api-spec/api/indices.settings.json

{
  "indices.settings": {
    "documentation": "http://www.elasticsearch.org/guide/reference/api/admin-indices-get-settings/",
    "methods": ["GET", "PUT"],
    "url": {
      "path": "/_settings",
      "paths": ["/_settings", "/{index}/_settings"],
      "parts": {
        "index": {
          "type" : "list",
          "description" : "A comma-separated list of index names; use `_all` or empty string to perform the operation on all indices"
        }
      },
      "params": {
      }
    },
    "body": {
      "description" : "The index settings to be updated (when using the PUT method)"
    }
  }
}