Apple will begin to warn Mac users this week that support for the 32-bit application in macOS is coming to an end. Ars Technica notes that Mac users running macOS 10.13.4 will receive a one-time alert tomorrow that will be activated once a 32-bit application is opened. The alert simply reveals that an application is "not optimized" for a Mac, but the application will still work for the time being.
Apple has not yet revealed exactly when support for 32-bit applications in macOS will cease completely, but the company already warned developers about the change and said that High Sierra would be the latest version compatible with 32-bit applications "without compromise". Apple has been making the transition to 64-bit hardware and software for more than 10 years, and the company has stopped accepting 32-bit applications in the Mac App Store.
Apple 32-bit application alert