Privacy-related requests. Because we don't delete entire account histories [1, 2], we do other things to help. That might include randomizing a username or otherwise renaming the account, deleting or redacting sensitive information, reassigning particular posts to random usernames, and more. We try to find a fair balance between the different interests involved; individuals' need for protection, other commenters' interest in having the context of their comments preserved, the community interest in preserving its archive, and so on.
I don't know what you mean by "incredibly problematic" etc., but if you think we should send such users away without recourse, that's an extreme position which a large proportion of the community would disagree with you about, especially as the pendulum of opinion has been swinging in the pro-privacy direction for years now. People frequently flame us for not doing more to delete history.
Just to repeat what I've already said a couple times today, we don't make any distinction between users who make these sorts of requests. We try our best to be as consistent as possible, and whether they're a YC founder or not doesn't enter into it.