Computerworld - https://www.computerworld.com/article/3268645/apple-mac/what-is-the-not-optimized-for-your-mac-warning-message.html#tk.rss_all
Mac users running the recently-released macOS 10.13.4 update may see a new alert message when they login and launch apps this morning, as Apple begins keeping its WWDC promise to push developers to upgrade their apps to 64-bit.
What is this alert?
Starting around midnight local time between 11-12 April, Macs running the latest OS version begin offering up a warning message the first time their users launched a 32-bit application.
32-bit app alerts are coming to macOS 10.13.4.