Diablo Immortal has been available for pre-registration since its announcement on November 2, 2018.
Ever since, you would only find the Diablo Immortal app by typing the title in the search box.
Even though you could pre-register by searching for the app, Diablo Immortal was not listed in the Pre-Registration Games list within the Google Play UI.
Diablo Immortal was added to this list somewhere between Friday, June 26 and Saturday, June 27th (noon) — range of time since last I checked Google Play.
It can be seen in the Android Emulator for PC: BlueStacks — which emulates a Samsung Galaxy S10.
It can also be seen in my normal Samsung Galaxy S10 phone.
As of June 28, Diablo Immortal still doesn’t show up in Google Play searches on Android devices below Android 10. Hopefully, that changes soon.
Recently, a Google Play update allowed pre-registered players the option to auto-download Diablo Immortal when it becomes available.