For companies with status or news feeds, it's a common pattern to use dismissible feed items to educate users on changes or drive important user actions. In this case, Github is using an embedded feed card at the top of the feed to educate users about some changes to their feed and to solicit product feedback.
On the right railing of the dashboard, Github has created an optional communications channel for promotional announcements, such as their upcoming event. These cards are dismissible so they do not unnecessarily annoy users who are not interested or take up key real estate. The dismissibility and the CTA also gives the Github team signal on engagement to train and refine any user relevance models for this and future promotions.