Just this past Friday (from what I can tell), Amazon announced the end of the Underground or "Actually Free" program that started in 2015. There are over 3700 games alone in the program. Check https://techcrunch.com/2017/04/28/amazon-is-shutting-down-its-underground-actually-free-program-that-gives-away-free-android-apps/ for the best details that I was able to find.
To summarize - no new apps will be accepted after the end of this month (May 31, 2017). The underground app will stop allowing access to the underground on Android phones and tablets this summer but will still allow access for Amazon/kindle fire tablets. The complete shutdown is due in 2019.
I contacted Amazon on one question. What happens to apps/games that you have "purchased" but do not currently have downloaded? Normally, downloading isn't an issue. If you have purchased it (free or cost), it's yours and you can download, remove, download as often as you want on as many devices as you want. That's not true. Come summer or whenever they drop that kind of support for the Android devices, you will not be able to download any Underground app or game that may currently be in your library.
That stinks! They have always pushed that once it's yours, it's yours and you will keep it. Now they're just telling us that we've wasted our time getting these apps.
If you have it already downloaded, it will continue to work so long as you keep it on that device. New devices are in the same boat as removing and wanting to reinstall - tough luck!
Why would anybody offer new apps now since it's been announced that new apps won't even be accepted after May 31?
They've also said that new Amazon devices after those currently on the market won't have the underground built in as it is now.
I am not a happy camper. I have 838 games and 1 app from the Underground! Most are series of games that play the same but have different images, such as DifferenceGames series of Hidden Differences or another of Hidden Scenes!