We're always working to improve and expand Khan Academy, including by adding more concepts to learn!
Whenever there are updates to course content that could affect your Mastery percentage, you’ll see a “New content added!” notice in the course, with a link to the Content Updates page. There you’ll find more information on what specifically updated so you can better understand what’s changed in course content.
You can also hit "Follow" in our News and Updates community forum. You'll see content updates there amongst other relevant news.