It's always admirable when a developer supports older operating systems; but there comes a time of diminishing returns when the effort to keep software working on old versions becomes disproportionately expensive compared to the ever-dwindling number of decrepit devices that still use an app. Such is the situation with Dropbox, which just declared the end of support for Android 5.x and older.
The message above was recently posted with the release of v198.2.2, confirming Dropbox will not be releasing any more updates with support for Android 4.4 and 5.x. It goes on to suggest upgrading to Android 6 or later — of course, most people probably would have done that already if it were an option.
Dropbox stopped sending out updates supporting 4.4/5.x with the release of 190.2.2 back on May 19th. Every release after that, and some of the betas released up to that time, were set to require Android 6.0 Marshmallow and above.
Since Android 5.0 Lollipop was released just over six years ago, and 4.4 KitKat is coming up on its seventh anniversary, most devices that would still be running those versions are at least 7 years old. Most people would have moved on from phones of that age, but it's not unheard of to still have some ancient tablets floating about. On top of that, many devices live out their last days as components in DIY projects.
According to the most recent distribution chart, 4% of devices still use KitKat, and another 9.2% are running Lollipop.
If you have a device that still runs one of the newly unsupported versions, keep in mind that the current Dropbox version should continue working normally for some time to come. Those devices simply won't get any new app updates, which means they won't see new features or bug fixes, and may be left vulnerable if new security holes are discovered. The service should continue to work with this version until Dropbox makes a breaking change to its network protocols or sign-in methods.
If you need to download the final release with support back to Android 4.4, the Play Store should still be able to serve it, or you can pick it up from APK Mirror.