What team did you report this issue to? I think this is more about the Slack Google Drive plugin's behavior than Google Docs per se. If someone with permission to view the doc takes a screenshot of the first page of the doc and sends that to you, there's nothing really that Google Docs can do to stop that. That's analagous to what's happening here. The Google Docs Slack integration is what's sharing the preview image.

I agree that it's poor behavior and potentially could be part of a critical security compromise. But you'd need to get a hold of the right team. The Google Docs core engineers probably don't care and couldn't fix it anyway.

I don't know for sure (and can't check now since I no longer work there), but my impression at a former job that made heavy use of Slack and Google Docs was that in a corporate setting with Google Enterprise or whatever it's called, the Slack integration was far more cautious about showing previews. It even alerted you if you posted a document that not everyone in the channel could view, and gave you the ability to grant them access right then and there. IIRC, previews were hidden if you didn't have access. I don't know if that is a different plugin or just better behavior in a corporate setting.