As announced last year during the WWDC (Worldwide Developer Conference) event, Apple will deprecate support for 32-bit apps in future versions of its macOS operating system for Mac computers.
With the macOS High Sierra 10.13.4 software update released last month, Apple implemented a tool for application developers to test their apps for 64-bit compatibility as 32-bit deprecation is upon us with future macOS releases, and now it looks like users are being warned about 32-bit app compatibility as well.
How To Open 32bit Apps On Macos High Sierra 10.12
'To ensure that the apps you purchase are as advanced as the Mac you run them on, all future Mac software will eventually be required to be 64-bit. Apple began the transition to 64-bit hardware and software technology for Mac over a decade ago, and is working with developers to transition their apps to 64-bit,' said Apple.
Some big apps out there need to be updated; others will disappear
If you updated your Mac to the macOS High Sierra 10.13.4 release, the next time you'll open your favorite apps, you'll see a warning that the app isn't optimized for your computer, which means that it hasn't been updated recently by its developers to use 64-bit processes.
Likes in the case of the alerts received by Apple Watch wearers for older watchOS apps, this is a one-time warning that you won't see again. However, Apple says that you can continue using any 32-bit apps you might have installed on your Mac, which won't have any adverse effects on your computer or the data contained on it.
Some big apps out there aren't optimized for macOS 64-bit operating systems, including Valve's Steam Client, as you can see from the attached screenshot, and others will start disappearing from the Mac App Store. As such, application developers need to update their 32-bit apps quickly, or they won't work on future versions of macOS.
Apple also reminded developers today that all newly submitted apps in the Mac App Store must support 64-bit technologies, as well as that starting June 2018 all existing apps and updates must also use 64-bit processes or they won't be accepted. More details can be found here and here.
How To Open 32bit Apps On Macos High Sierra 10.13
Install the 32-bit apps you want into this new virtual machine. I made a video of this entire process—I edited out the boring parts but left all the essential steps. The end result is the 5-minute video below. For this video, I chose macOS High Sierra as the older Mac OS, but you could install Mojave, Sierra or almost any older Mac OS. Apple actually started to warn people about the change with macOS 10.13 High Sierra. If you launch a 32-bit app in High Sierra, an alert pops up to tell you that your app needs to be updated.