The latest Insider Preview releases of Edge sport a new feature called "Discover". It appears are a new button next to the menu button with a glowing star. Clicking it displays a huge pane with some content you would find interesting to learn, of course in opinion of the browser.
The new button is actually part of the sidebar. It is one more web app bundled with it. However, there is a huge difference from other sidebar buttons.
If you hide the sidebar in browser settings, the new button remains visible in the toolbar and doesn't go away.
More than that, it opens the "Discover" pane just on hover. Considering that it now takes place of the menu button, it is not hard to predict that lots of users will accidentally hover over it, or click it, just due to their muscle memory.
The new feature doesn't expose any option to disable it in Edge's settings. However, you will find a setting, which is enabled by default, to automatically open "Discover" if the browser found something similar to what you are browsing. So there is a great chance that you will deal with native popups. 
Finally, I didn't find any flag to disable Discover on the edge://flags page, at least in the Dev version of the browser.
Obviously, such changes will draw negative feedback even from hardcore Edge fans. Unlike the screen split feature, and the upcoming major browser redesign, the new addition doesn't look much useful. It is totally unexpected to see such a change in the browser when just a few days ago you learned that Microsoft is working on a minimal toolbar experience to reduce users' distraction.
Support us
Winaero greatly relies on your support. You can help the site keep bringing you interesting and useful content and software by using these options: