macOS 10.13.4 now warns you when you open a 32-bit app
As of June 1st 2018, the Mac App Store will require all apps to support 64-bit architecture. In preparation for this change, after updating to macOS 10.13.4 for the first time, you may encounter an alert when opening a 32-bit app. This is because the system will no longer automatically launch these applications, and users will need to take action to continue using them.