I just updated my Pixel XL2 to Android 9 and I was experiencing new UI designs almost everywhere. After navigating to Digital Wellbeing Beta which works only on Android 9, I recognized that Google Play beta opt-in page got a new Material UI. It looks fine on desktop but it doesn't have proper padding on mobile.
The link to the google play was moved from the bottom. Additionally, the link for the opt-out became a proper button and now it will be easier for newcomers to find it.
In order to see this change, you don't need to install anything - it should be available for everyone. You can try it with any app from our catalog of Android apps for beta testing.
Did you fi find any other changes in the Beta testing process?