Apple has been trying to get rid of 32-bit programs and applications for some time now, and not only in iOS.
In this regard, Apple will begin to notify macOS users that they are using 32-bit programs, and that macOS High Sierra will be the last version of the operating system with their support. When macOS High Sierra 10.13.4 is released (the first beta version of which was released today), all 32-bit programs will receive notifications that they will not be supported in future versions of the OS.
It should be noted that notifications will appear only once. So if you open the program, close it, and then open it again, you will not see the notification a second time.
Apple continues to get rid of 32-bit programs quite aggressively. From January this year, all new applications in App Store must be 64-bit only. 32-bit applications Apple no longer accepts. Moreover, all software and application updates must now only be 64-bit.
As in iOS, soon 32-bit programs won't work at all on macOS. In the meantime, users have time to find replacements for their favorite programs and applications, or hope that the developers will release their 64-bit updates.