Today, Amazon noted that since Underground Actually Free began, its app store has expanded to include over 800,000 apps and games. However, it looks like the company is going in a different direction for its app store. It stated that after May 31, developers will no longer be able to submit apps or games to the Underground Actually Free program. Any such apps that were published in the program prior to that date can still be updated until the end of 2019, and developers will still be paid for every minute users spend on those apps until their contracts run out.

For Android owners, access to the Underground Actually Free store will end sometime in the summer of 2017, but those users will still be able to play any apps downloaded on their device from that program. If you own an Amazon Fire tablet, you can still access the Underground Actually Free store and download apps and games until the end of 2019.
Read more at Android Authority

I've used their app store a LOT for free apps, so this is really disappointing for me. I think this is going to hurt Fire tablet sales once it's completely shut down.