It looks like it's missing from `ReindexOnServer(...)` on the high level client. I'll open an issue to add it.

In the meantime, you can submit the request using the lowlevel client exposed on the high level, and still get back a high level response

```c#
var reindexResponse = client.LowLevel.Reindex<ReindexOnServerResponse>(@"{
      ""source"": {
        ""index"": ""twitter"",
        ""_source"": [""user"", ""_doc""]
      },
      ""dest"": {
        ""index"": ""new_twitter""
      }
    }");
```

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