In sad news from the developer Team Dakota, Project Spark is no more.
As of yesterday, the game is no longer available for download from the Xbox Marketplace or the Windows Store, and the online services will be killed on the 12th of August this year.
As the game went to a "free incubation model" last year (i.e. free to play), it appears that the development staff have been reassigned, so the one silver lining here appears to be that there will be no job losses as a result. If you bought the game at a retail store between October 5th 2015 and and 13th May 2016, the forum post states that your Microsoft account will be credited, so no monetary loss should ensue.