Microsoft announced its plan to discontinue the Ad Monetization platform in a brief post on MSDN (which I saw via MSPoweruser.com).
"his decision was made primarily because it is no longer viable for us to continue operating the product at the current levels," says the company's announcement.
Developers using the platform will be paid using the regular payout schedule through May 31. Microsoft is advocating that UWP developers make a transition to another ad platform.
As Microsoft officials acknowledged publicly last year, Microsoft's original plan to try to convince developers to create Universal Windows Platform (UWP) apps that would be updatable and distributable solely via the Microsoft Store failed to materialize. Many developers didn't see Windows -- growth of which is slowing -- as a platform for which it would be worth the time and effort to write new UWP apps and/or adapt their existing Win32 ones.