Yes, if it was at one time public in the internet, it will surely be forever in the internet. That's correct, but I don't think you should support that behaviour. I think the easiest idea would be to remove that stuff if someones asks for that. It should not happen often enough to hav significant influence on the time needed to moderate the site. That makes everyone happy: users can remove they infos if needed, and you don't have to change the site's programming.
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