macOS 10.13.4 is alerting users that their 32-bit apps will soon be unsupported. A new dialog will only appear once for each app, when it's launched, and signals a transition to 64-bit apps that started with iOS.
The move is not surprising and is something we've reported on before. But Apple has quietly upped the ante, putting users and app developers on notice that older apps, that haven't been updated, will soon stop working.
With WWDC only a few weeks away, and the next version of macOS, as well as iOS, tvOS and watchOS, expected to be revealed and made available to beta testers it's possible we'll see this change added to the as yet unnamed successor macOS High Sierra. And with rumours abounding that Apple is looking at a world of universal apps, where iOS and macOS programs can work across all of Apple's computing devices, bringing everything up to 64-bit compatibility is a step along that road.
Apple has provided a support note about the move.