Microsoft and Team Dakota will be ending support for Project Spark on August 12. Effective immediately, the game/creation incubator will no longer be available from the Xbox or Windows Stores.
If you're not familiar with Project Spark, it's a fairly unique title. It allowed players to create their own worlds and their own games. The game was free-to-play and back in September, the firm announced that all downloadable content would be free, but at the same time ended active development on the game; however, there was no sign of it actually shutting down at the time.
For anyone that purchased the Project Spark Starter Kit that was sold in retail stores, a credit will be issued to their Microsoft accounts. For those that purchased and redeemed the code found inside of a retail copy of Project Spark after October 5 (the date that downloadable content became free, and active development on the game ended), they will also receive a credit to their accounts, which can be used to buy content from the Windows or Xbox Stores.
As of August 12, online services will no longer be available for the game. This means that users will "no longer be able to download user-generated content or upload their own creations". It's worth noting that if you want to continue to have access to your creations, you should make sure that they're locally stored and if not, download them.