Since 2002, iPhotos and Photos on the Mac have always offered the ability to create books, cards, and calendars, as well as to order prints of favorite photos. With the recent update to macOS 10.13.6, Apple quietly announced that it would no longer offer Photos Print Products after September 30, 2018. The announcement came in the form of a dialog that appears when selecting any of the File > Create options in Photos:
The project extensions described in the dialog have actually been available for some time, primarily for those who wanted an alternative to Apple’s Photos Print Products.
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At this time, clicking the Open App Store button on the dialog launches the Mac App Store and displays product extensions from a handful of vendors that provide printed products from Photos (see image below):
It’s unknown why Apple has decided to drop “in-house” printed Photos products, but it is most likely due to the explosion in the use of iPhones and iPads to view photos. That reduced the demand for printed photos and books, and as a result, the ability to create printed projects was never added to the iOS version of Photos. The current macOS 10.14 “Mojave” beta also directs users to Project Extensions in the Mac App Store.