Apple has been working to phase out 32-bit apps for quite some time now, and not just on iOS.
In that continued effort, Apple will start to notify macOS users that they are using 32-bit apps, and that macOS High Sierra will be the last version of the desktop operating system that will support 32-bit apps at all. So, once macOS High Sierra 10.13.4 (which is now in its first developer seed) arrives to the public later this year, any 32-bit app that is opened by a user will also come with a pop-up notification that will let the user know that 32-bit apps will not be supported in future versions of the OS.
It is worth noting that the notification for an app will only pop up once. So if you open an offending app, close it, then a week later try again, you won’t have to worry about the notification telling you about 32-bit app support getting the axe in subsequent updates.
For Apple, it continues to move away from 32-bit apps in a pretty aggressive fashion. As of January of this year, all new apps that are submitted to the App Stores must be 64-bit. Apple simply doesn’t accept new 32-bit apps any longer. Moreover, all apps and app updates that are submitted must be 64-bit as of June 2018.
As is the case with iOS, once 32-bit app support is dropped entirely, those apps will not work on macOS at all. At that time, users will need to find replacements for those apps if the developers have not updated to 64-bit options.