When Apple introduced the ability to publish apps to the iOS App Store in 2008, it was a very different landscape from what we have now. Back then you had either apps that you published for free or ones that were paid up front. Now, free apps are far more common than paid up-front apps.
In 2010, Apple introduced a new product, the iPad, which allowed for more opportunities within the App Store. With the introduction of the iPad you had two options, create a universal app, one that would work on both the iPhone and the iPad, or create two separate apps; one built for each platform.
While the possibility to build two distinct apps remained for a while, the introduction of the Apple Watch and the Apple TV have made the idea of creating distinct apps for each platform a bit harder to accomplish. The interfaces should be tailored for each platform, but the app itself would likely be shared amongst iOS and iPadOS.
Last year with the introduction of macOS Catalina, there was a new way to distribute your existing iOS apps, to macOS, through a project called Catalyst.
With free apps there are a number of different strategies for supporting free apps. These can be, ad-based, subscriptions, or in-app purchase. It has become more and more common for the latter two of these to be used. With in-app purchases, if you built an application for both iOS and macOS, using Catalyst or native frameworks, you would have to create two different in-app purchases, because they could not be shared between the platforms.
With the introduction of iOS 13.4 and macOS Catalina 10.15.4 you will
be allowing customers to enjoy your app and in‑app purchases across platforms by purchasing only once. You can choose to create a new app for these platforms using a single app record in App Store Connect or add platforms to your existing app record.
This is a huge change for the App Store and the distribution of apps in general. Users have been requesting the ability to purchase an app once and have it work on all of their devices. While this works for users, this can have some implications for developers.
While the option to distribute a single application to all of the platforms is optional, it is likely something that users will quickly come to expect from developers. Yes, there are tools like Catalyst for macOS, it is still not at its full maturity in terms of having iOS apps ported to the Mac look and behave like native macOS apps that use AppKit.
This can have some ramifications for the developer. The first being that this can easily cut into profits for a developer. For larger companies, this may not be a big problem, but for the smaller independent developers this can have a huge impact.
With the pressure to make your application available on all platforms, and in-app purchases being good across all platforms, this will likely reduce the income for developers.
There are some developers who have wanted to have universal in-app purchases available because they want their users to be able to have the same experience on all platforms, plus users also question why they have to make the same purchase on multiple platforms. Therefore, this will be a great addition for both users and developers.
In-App Purchases on watchOS
Starting with watchOS 6.2, developers will be able to provide in-app purchases directly from watchOS. This will have a huge benefit to the watchOS platform as developers will not need to have users use their paired iPhone to perform in-app purchases, but instead have it possible to purchase them directly on the Apple Watch. This should provide a better experience for Apple Watch users and the in-app purchase workflow.
While the addition of universal apps as well as universal in-app purchase will create a better experience for users, it could have some ramifications for developers in that they will be expected to support universal in-app purchase, which developers may want to do, as well as supporting universal app purchase, which may reduce their income.
I cannot say that this is not altogether unexpected, because it is something that both users and developers have been asking for for a while. It may take some time for applications to come to support universal in-app purchases as well as universal app purchases. This should be available starting with iOS 13.4, macOS 10.15.,4, tvOS 13.4, and watchOS 6.2.