Apple has been warning developers for some time that 32-bit apps would be deprecated on macOS, as they have already been on iOS. macOS High Sierra is now displaying a warning whenever you launch a 32-bit app, informing you that the app is “not optimized for your Mac.”
If you click OK, the app will launch, and if you click Learn More, you’ll be taken to an Apple technical document explaining this change. You can continue to use these 32-bit apps, and, as Apple says, “Using 32-bit software has no adverse effects on your data or your computer.” However:
The 64-bit transition for macOS and macOS apps is still underway, so final transition dates have not yet been established. But now is a good time to check with the software developer to see if 64-bit versions of your favorite titles are available.
So it’s a good time to check if the apps you depend on have updates available.
There’s an easy way to find all the apps on your Mac that are 32-bit. Choose the Apple Menu > About this Mac, then click System Report. In the System Information app that displays, click Applications in the Software section, and you’ll see a list of apps. Click the 64-Bit header to sort by No or Yes. You’ll see a number of apps, including some by Apple.
Again, there’s nothing to worry about right away, but it’s a good idea to check if the apps you use have been updated. Many apps automatically check for updates, but not all; for example, my ScanSnap scanner software does not do this, so I had to go find a new version on the vendor’s website.