You Can Still Get Circular Folders
In May, some iOS 9 users discovered a bug that allows you to change rectangular user-created folders into circular user-created folders. This bug/feature still exists in iOS 9.3.5.
If you're interested, here's how to get circular folders on your iPhone 6s.
First, you'll need a 3 x 3 pixel background. Head to that link, choose a color you like and save it to your Camera Roll.
After that's done head to Settings > Wallpaper > Choose a New Wallpaper. From there, select Camera Roll/My Photo Stream and select the wallpaper you just saved to your iPhone 6s. Apply the new wallpaper to your Home Screen.
Go back to your home screen you'll notice circular folders. Again, it only works with user-created folders so some of them will still appear as rectangles.
It's not clear if Apple will patch this feature/bug up with a future update.