There are probably a ton of apps on the Google Play Store that haven’t been updated since their release, and possibly for years. Maybe the developer has forsaken the app, or maybe the app is simple enough where despite the various updates to Android, it still works. However that will change with Android P.
According to a report from Android Police, it seems that with Android P, Google will block apps built for Android 4.1 or lower from running on the phone. This means that users who are still using these very old apps will not be able to use them if they decide to make the upgrade to Android P.
We suppose this makes sense as some apps, despite being functional, might have some security vulnerabilities or exploits that can be taken advantage of that could have been fixed had they been updated. Or it is also possible that apps that are Android 4.1 old might not be optimized for modern handsets in terms of features, UI, resolution, and so on.
Google is not alone in this as Apple had previously mandated that developers update their apps to 64-bit if they wish to remain in the iTunes App Store. Android P is currently available as a developer preview where its release will most likely take place towards the later part of the year.