Just to follow-up to my post: Apple responded to my case I opened with them. They confirmed that there is no official documentation regarding the itms-apps URL scheme, and that there is no guarantee about engineering side changes. But they suggested I use the format itms-apps://itunes.apple.com//developer//id. The is the code that matches the developer's apple ID registration. For the United States, for example, it's "us". I confirmed this also works with iOS 12.1.