Following on from the changes imposed as part of iOS 11, macOS High Sierra, which is about to drop for Mac users, has discontinued support for older 32-bit apps as well as a bunch of other software.
Progress is relentless and Apple has rarely let a little thing like backward compatibility stop them from marching ahead. While some of their other transistions, such as the move from Power PC processors to Intel, came with some transitional support, some have been more abrupt.
macOS High Sierra will only support 64-bit applications. The transition has been on the cards for a while and, if you get your apps through the macOS App Store, you'll likely see a bunch of updates flow through over the coming days.
Some older apps will also stop working. Microsoft won't support Office 2011 for the Mac on High Sierra and you'll need to be running Office 15.25 or later for High Sierra support.
I've not yet hit any compatibility issues with High Sierra - I've been running betas for a while now. But if you've found an app that breaks in High Sierra it would be great to share that information with the rest of the Lifehacker family.