Apple has announced that apps can now be published as "unlisted" to the App Store.

Apps that are unlisted will only be accessible via a direct link and cannot be discovered through a typical search, categories, recommendations, or chart position.

Here are a couple of provided examples where an unlisted app could be useful:

Limited audiences (such as part-time employees, franchisees, partners, business affiliates, higher-education students, or conference attendees)...