The warning says Bing may demote the site or delist the site that is using keyword stuffing. The new section was added sometime yesterday to their guidelines document.
Here is the new keyword stuffing section:
When creating content, make sure to create your content for real users and readers, not to entice search engines to rank your content better. Stuffing your content with specific keywords with the sole intent of artificially inflating the probability of ranking for specific search terms is in violation of our guidelines and can lead to demotion or even the delisting of your website from our search results.
As you can see, Bing is not playing games. If you use keyword stuffing, it can “lead to demotion or even the delisting of your website.”
Hat tip to +Menashe Avramov for informing me about this update.