Apple today reminded developers about upcoming changes being made to the Mac App Store. Starting on June 1, all new app updates submitted to the Mac App Store must support 64-bit.
If you’re using an old 32-bit app on a Mac, macOS will display an alert to let you know that it may not run optimally and that you should check with the developer to see if an update is available.
The alerts appear only when a 32-bit app launches, and will have a Learn More button that takes users to an Apple KnowledgeBase article that explains the 32- to 64-bit transition.
The alerts are part of the macOS 10.13.4 update and are set to trigger at midnight on Thursday. April 12. If you do not have version 10.13.4 installed, you will not see the alerts, but they will appear once you update your Mac.