ODK Collect currently supports Android versions down to 4.1. The 4.x Android versions are very different from modern ones so this has caused a significant developer and QA burden.
Android 5 was released in 2014 and only about 6% of current Collect users use versions of Android <5. The biggest change coming to the next Collect version v1.26 is the start of the scoped storage migration (https://forum.getodk.org/t/collect-will-need-to-stop-using-sdcard-odk-for-files/24159). This is not relevant to users on Android 4.x so we have proposed simultaneously dropping support for Android 4.x which @TAB agreed on during today's call.
This means that Android 4.x users will continue to see Collect in the Play Store but that the latest version they will be able to update to is v1.25.1.
On the call, @ggalmazor asked about also dropping support for Android 5 and 6. That will be the next big milestone because once the lowest Android version supported is 7, a lot more Java 8 features will be available. @tomsmyth also asked about writing out a policy for dropping Android version support so that we don't have to revisit this question periodically.
Each of Android 5 (mostly 5.1) and 6 represent about 11% of Collect users. That's 22% of users combined who would no longer get Collect updates. That feels like a lot and I am not in favor of dropping updates for them now.
Only dropping Android 5 support has very little benefit so I think the next decision to make will be when to drop both Android 5 and 6. After that it will be much easier to develop a standard policy because the nature of Android and Android devices has changed. Before Android 7, it was generally impossible to get Android updates so if you got an Android 5.1 device, it would be an Android 5.1 device forever. More and more, it is possible to get Android release updates.
I propose we consider the transition to Android 7+ carefully based on number of Collect users affected and upcoming large features so that we can bring as much of the user base forward as possible. After that, we can develop a policy based on when new Android releases come out.