The beta versions of macOS 10.3.4 were reported to include warnings when you started a 32-bit app. From the release notes:
To prepare for a future release of macOS in which 32-bit software will no longer run without compromise, starting in macOS High Sierra 10.13.4 a user is notified on the launch of an app that depends on 32-bit software.
I've been using the released version of 10.3.4 for a few days, but I haven't seen any notices when I run 32-bit software. Was this feature removed? Or are there special conditions that need to happen before it appears?
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.