High Sierra 10.13.4 is the last MacOS version that supports 32-bit software. From next version of MacOS only 64-bit apps will be supported. So, lot’s of MacOS software will stop working.

Apple ha begun warning MacOS users

Both Ars Technica and The Verge reports that Apple has begun warning MacOS users about 32-bit apps.

Apple’s High Sierra 10.13.4 is the last version with full support for 32-bit apps. Those apps that is not optimized for running on 64-bit will be doomed totally. These 32-bit made apps needs to be updated by the developer to improve compatibility now. If it’s not updated by the time next MacOS update arrive, they wont work at all and  there will be a error message saying when opening an app that is not 64-bit. For now, only a message that warns about no support from next update appears when running 32-bit app on Apple’s High Sierra 10.13.4. But from the next version it’s no mercy.

32-bit apps will work on High Sierra but in the next MacOS version they won’t

The app will continue to work, but it’s uncertain how long. Apple has previously announced that MacOS High Sierra is the last release supporting these apps “without compromise” and 32-bit apps are no longer accepted in the Mac App Store.

On June 2016, Apple began to inform the iOS 10 users about the same. Now it is the MacOS user’s turn.

Source
Ars Technica
The Verge