The company updated the Aperture application description on the Mac App Store with the following statement:
When Photos for OS X is available this spring, Aperture will no longer be available for purchase from the Mac App Store.
Apple revealed that it would be stopping development on Aperture in June; however, the company didn't indicate that it would be removed from the App Store.
If Aperture is important to your workflow, make sure you have it in your list of purchased applications. It should be available to download from the purchased tab even after it's pulled from sale.
If you are looking for another professional grade photo application to work with, Adobe has been 'doubling down' on Lightroom and recently released an update with an integrated Aperture and iPhoto import utility.
You can purchase Aperture from the App Store for $79.99.