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The article doesn't seem to mention if the sensitive information from the email was marked as such.

Why would it need to be? Surely Hetzner and the like would be familiar with how sensitive it could be.

If you consider nicknames sensitive you are gonna have to mention that, because a lot of people wouldn't.

Hetzner, as a hosting provider, would likely be well aware of the workings of IRC networks, so I doubt that.