Beginning June 1, 2018, all new app updates submitted to the Mac App Store had to support 64-bit. Apple is expected to phase out support for 32-bit apps in macOS 10.15, which will almost certainly be previewed at the 2019 Worldwide Developer Conference in June. (It’ll kick off on Monday, June 3.)
This means that means some older apps that haven’t been updated in awhile will cease to work. Thankfully, it’s easy to tell if an app is 32-bit or 64-bit.
As developers optimize their apps for 64-bit compatibility, Apple is notifying customers when they’re using an app based on 32-bit technology via a one-time alert that appears when you launch a 32-bit app. In macOS Mojave, this alert appears once every 30 days when launching the app.
To check if an app is 32-bit or 64-bit, from the Apple menu, choose About This Mac, then click the System Report button. From the system report, scroll down to Software in the sidebar, then select Applications. When you select an individual application, you will see a field titled 64-bit (Intel). “Yes” indicates 64-bit; “No” indicates 32-bit. If you’re using macOS Mojave (and you should be), select Legacy Software in the sidebar to see all applications that haven’t been updated to use 64-bit processes.
Alternately, you can use the free 32-bit Check application (https://eclecticlight.co/32-bitcheck-archichect/), which lets you save, print, and filter the list. With it, you can check the folders of your choosing, and can check just apps, or all bundles including plugins and other executable code.