LG and Samsung have officially ushered in the era of using an unusual aspect ratio to put a big screen on a small phone, and Google wants to be sure that Android app developers are ready for it. To that end, they've made a blog post asking developers to update their apps to support the new 18:9 aspect ratio that LG and Samsung's flagships are aiming to popularize in the coming year or so, and have explained exactly how to do it. So long as everything else about the app is in place to support the new aspect ratios with no issues, raising the maximum aspect ratio of your app is as simple as modifying a single line of code, and the result, should everything go right, is an app that looks perfectly natural on an elongated screen.
To be specific, developers should seek out the "android.max_aspect" code in their app. If such code is not present, then the app will, by default, not support the larger aspect ratio being pushed by the two South Korean flagship devices. If it is not present, developers can add it in under the app's <application> code element. If it is already in place, developers simply have to modify it. The value should end up looking like "<meta-data android:name="android.max_aspect" android:value="2.1" />". So long as that code is present and properly implemented, just about any app can be made to support the new aspect ratio on the block, though developers are on their own in fixing any bugs that arise as a result of essentially elongating their app window, such as visible elements that were hidden before, or elements moving out of place.
While it's far too early in the game to tell if 18:9 aspect ratios will become the new normal, the way the facts stand right now is pretty simple; two of the world's most popular smartphone OEMs have put that aspect ratio on their top-shelf devices, meaning that app developers could be panned or even shunned by users if they don't support the full glory of the new screens. While Samsung's Galaxy S8 and Galaxy S8 Plus include software that can automatically stretch some non-compliant apps and media, not everything is supported, and the LG G6 does not have such a software feature; this means that the likelihood of the feature becoming widespread is pretty low, leaving the onus to update squarely on developers.