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?