As new versions of the Android operating system have made their way into the mainstream over the years the API level has gradually increased either with major versions or selected point releases. In fact, the most recent iteration of the OS, Android 9.0 Pie, has a corresponding API level of 28. However, even though Android's APIs have grown and matured over the years, developers haven't necessarily coded their apps to take advantage of them for a variety of reasons, such as the necessity to add runtime permissions if aiming for release on devices running Android 6.0 Marshmallow or later.
Now, it appears that Google will be attempting to push things in the right direction with the introduction of Android Q. Last year, the company did foreshadow a number of measures in a blog post late last year, requiring that new apps target API level 26 (Android 8.0 Oreo) as of last month and that updates to published apps target the same API level from November 2018 onwards. On top of that, based upon a commit made to the Android Open Source Project, Android Q will warn users when they attempt to install apps that fall below these "provisional" thresholds.
While this is good news as far as newly developed or regularly updated apps are concerned, these measures don't do much in the way of bringing older or abandoned apps up to date. Furthermore, it seems that users will still be able to install such apps on their Android Q devices but will have to put up with tapping past a warning message every time one of these apps is launched. Of course, Google may yet adjust the reported threshold before the OS is released, so time will tell how strict the final implementation ends up being.
Update: This warning window has been seen previously on Android P; however, the minimum API level is being increased from 17 (Android 4.2.x Jelly Bean) to 23 (Android 6.0 Marshmallow).
Source: XDA Developers