As noticed by 9To5Google, a support webpage confirms the disappearance of the My Case section from the official online US Google Store is not an error of any sort, as the company no longer sells customizable phone cases. That includes older Live Cases for the Pixel 2 and 2 XL, even older Live Cases with NFC buttons for the original Pixel, Pixel XL, Nexus 5X, 6P, and Nexus 6, as well as newer My Cases designed for protective use of the Pixel 3, 3 XL, 2, and 2 XL.
The difference between the two programs is that Live Cases worked with companion live wallpapers to brighten your day and match the front and back of your phone in a number of interesting ways, while My Cases were more static but still significantly cooler than your regular handset protective accessory, letting you slap a photo of your choice to the rear of the Pixel 3.
Unfortunately, that's no longer possible, and although it's not entirely clear why Google decided to make this change, a few potential explanations come to mind. Since the timing seemingly corresponded with the Pixel 3a and 3a XL announcement, one could assume the company deemed the customizable accessories unfeasible for a family of four phones with two additional models on the way. Or maybe the My Case program was never that successful. Whatever may or may not have happened, Pixel 3, 3 XL, 3a, and 3a XL owners can still buy conventional fabric cases from Google... or settle for a third-party option to choose a different design or build material.