Today and tomorrow, macOS users will begin seeing notifications informing them that 32-bit apps will not be supported in a future version of macOS, Apple representatives told Ars.
Starting at midnight April 12, 2018 in the user’s local time zone, they will see the following message the first time they launch an app that only supports 32-bit in macOS High Sierra 10.13.4:
This app needs to be updated by its developer to improve compatibility.
Along with that message, they’ll see a “learn more” link that takes them to Apple’s support page on the subject with more information. The support page broadly explains Apple’s plans to “eventually” require all Mac software to be 64-bit. It also reiterates several statements Apple has made to developers in the past, like specifying that High Sierra will be the last version to run 32-bit apps “without compromise”: