Recent changes to the Android Open Source Project (AOSP) code commits suggest that Google is reconsidering which API Android developers will need to target to prevent users from seeing a warning that the app might not work properly. For now, Android 9 Pie has a built-in feature that provides users with a warning if an app they are using targets an API at or below Android 4.2 Jellybean. That shows up every time an activity from the app instantiates. The latest change, which is currently marked as provisional, moves that forward to API 23. The version of Android associated with that is, of course, Android 6.0 Marshmallow, which added underlying security features associated with permissions at runtime. If the AOSP change is pushed through to the system itself in a future update, developers will need to target Marshmallow at a minimum in order to avoid users seeing the warning message.
The addition of a warning to users has been part of a much more widespread effort to enforce policies regarding API targetting. Namely, it's intended to address security concerns in older API as well as to bolster efficiency and privacy across the platform. However, rather than enforcing a minimum target API across the board, the search giant is counting on users to encourage developers to change things. For new and current Google Play Store apps, the company requires that Android Oreo or later is targeted at API 26 but there is no minimum API target defined. Instead, users with applications that are designed to work with older, less secure versions of Android get a warning that their apps might not function properly. A link is also provided to contact the developer or check for updates. That, in turn, indirectly implies that the app's developer has not been keeping up with changes to Android's underlying architecture.
Since the change is marked as provisional, there's a chance that Google has still not decided to specifically target API 23 as its minimum before displaying a message. There's also no indication as to when this might appear on the user side of Android itself. The change is something that could be introduced alongside a smaller release without completely reworking the firmware. So it wouldn't necessarily require an update to whatever version of the OS comes after Android 9 Pie.