1) As a California LLC, it is unlikely that Ycombinator has a legal obligation to comply with a European data law. Under California law, though, you do have the right to request deletion of your personal information if you are a California resident: https://www.ycombinator.com/legal/.
2) Legal responsibility aside, if you email hn@ycombinator.com with your request they are likely to help you do whatever you would like. I think you are better off phrasing it as a polite request, though, rather than a veiled legal threat.