Y2K seems like eons ago! There is no one answer but anything around preparation for Y2K could probably be safely disposed of. It's been 9+ years. I'd keep Y2K audit or exam materials for the same time period as other audit/exam materials. If you had litigation or negotiations with service providers over anything Y2K related, then you would especially want to follow the direction of your counsel for proper retention.
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Opinions are mine not my employer's, and should not be taken as legal advice.