Starting at midnight tonight, Apple will begin warning macOS users that their systems will not support 32-bit apps for much longer. The first time users launch a 32-bit app, they will see a warning that says, "This app needs to be updated by its developer to improve compatibility." A link on the warning will take users to a page that explains Apple plans to require all macOS software to use 64-bit application architecture. It adds, "Apple began the transition to 64-bit hardware and software technology for Mac over a decade ago and is working with developers to transition their apps to 64-bit. At our Worldwide Developers Conference in 2017, Apple informed developers that macOS High Sierra would be the last version of macOS to run 32-bit apps without compromise."
Apple hasn't said exactly when it will end 32-bit support, but it has stopped accepting 32-bit apps in the Mac App Store. It could announce more details at its 2018 Worldwide Developer Conference (WWDC) in June.