According to a Google support page, the Google My Case program — originally called the Google Live Case program — is no longer active. With its shuttering, there is now no official way to create a custom case for your Google-branded smartphone.
Usually, you would find the Google My Case page listed on the accessories portal at the Google Store, but it is no longer there. The popular fabric cases are still available, however.
Google My Case would allow you to upload your own photos and artwork to then get printed on a custom phone case for your Google Pixel device. My Case evolved from the previous Live Case which would print you out a case featuring an image from Google Earth and then combine that with a live wallpaper of the same image.
Google has offered some sort of custom case program since way back in the days of the Nexus 6. With the dissolution of this program, Google is ending a nearly five year run of this kind of product.
Luckily, there are still plenty of other third-party companies one can use to customize a phone case. However, they won’t come from Google and you likely won’t get a custom live wallpaper to go with it.