Apple has begun the process of phasing out 32-bit applications on Macs.  With the recent release of macOS High Sierra 10.13.4, the first time users launch an app that does not support 64-bit they will see an alert that the “App” is not optimized for their Mac.  

32-bit App warning

This is the first of many warnings Apple plans to provide as ends support for 32-bit apps on the Mac, as it did on iOS devices with the release of iOS 11. Apple has confirmed that macOS 10.14 Mojave, set for public release in the fall, will be the last version of macOS to allow 32-bit apps to run, but it will include more “aggressive” warnings about their use before they are phased out entirely.