From June 1, 2018, all new app updates are sent to the Mac App Store had to support 64-bit. Apple is expected to discontinue support for 32-bit apps in MacOS 10.15, which will almost be previewed at the 2019 Worldwide Developer Conference in June. (It will start on Monday, June 3.)
This means some older programs have not been updated in a while, will stop working. Fortunately, it's easy to tell if an app is 32-bit or 64-bit.
As developers optimize their applications for 64-bit compatibility, Apple informs customers when they use 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 the app is started.
To check if an app is 32-bit or 64-bit, from the Apple menu, select About this Mac, and then click System Report button. From the system report, scroll down to Software in the sidebar and then select Programs. When selecting a single program, you will see a field titled 64-bit (Intel). "Yes" indicates 64-bit; "No" indicates 32-bit. If you use macOS Mojave (and you should be), select Legacy Software in the sidebar to see all applications that have not been updated to use 64-bit processes.
Alternatively, you can use the free 32-bit checker (https://eclecticlight.co/32-bitcheck-archichect/), which allows you to store, print and filter the list. With it, you can check the folders you choose and can only check programs, or all bundles, including plugins and other executable code.