Certainly not deleted, but they are being given a weak penalty in the hope that only the most HN-appropriate (i.e. intellectually interesting and/or providing significant new information) posts will rise to the top. We take the penalty off when we see one of those, and upvotes can overcome it anyway—the penalty is deliberately weak to allow for that.

Edit: here's an example on the front page right now: https://news.ycombinator.com/item?id=8461304.

The intention is not to suppress an important topic but to hold the countless me-too posts at bay—the ones that don't add anything significant. When the massive wave subsides, we'll remove the penalty. For example, there was a period where HN did this for NSA stories, but that hasn't been the case for a while.