Apple has started informing users that macOS will soon stop supporting old 32-bit apps in favor of 64-bit ones, a similar move to when Apple retired 32-bit architecture apps on iOS 11.
These apps will not be supported in a future version of macOS, Apple cautions. Starting April 12 in the user's local time zone, they will begin seeing the following message every time they launch a 32-bit app in macOS High Sierra 10.13.4:
"This app needs to be updated by its developer to improve compatibility."
Goodbye, 32-bit macOS Apps
The dialogue box also includes a support page link that details Apple's plans for all Mac apps to run in 64-bit architecture. Apple also mentions that macOS High Sierra will be the last version of macOS that'll support 32-bit apps "without compromise."
Even still, the company has yet to specify exactly when it's planning to halt 32-bit app support entirely. These apps still run and perform normally for now, but the aforementioned "without compromise" statement implies that they'll be less functional and useful once the next version rolls around.
A software update will most likely arrive in September or October, if Apple's release patterns are to be gleaned from. Expect Apple to reveal more details about the next version of macOS during this June's Worldwide Developers Conference.
Why Apple Is Halting 32-Bit Support
Apple is transitioning into 64-bit territory simply because apps built under this architecture are much easier to maintain, and perhaps because these apps are far more efficient than their 32-bit counterparts. 64-bit apps are also capable of utilizing more memory than 32-bit ones, meaning they are able to perform faster, and much more smoothly.
Apple has already begun rejecting 32-bit apps submitted to the Mac App Store. Even so, it's not likely Apple will immediately cut off support willy-nilly. Developers might still have a few months to work on their apps and transition into 64-bit, if they haven't already.
How To Know If My macOS Apps Are 32-Bit
Launching a 32-bit app will obviously trigger Apple's warning, but there's a much easier way to get a full list of all the 32-bit apps you have. Start by going to the Apple logo on the top left of the menu bar. Click it, then choose "About this Mac." In the "Overview" tab, click on "System Report." A new window will open. Under "Software" in the sidebar, look for "Applications," then sort the list by "64-bit (Intel)." If an app shows "No," that means it's 32-bit and needs to be updated.