https://arstechnica.com/gadgets/2018/04/apple-has-begun-notifying-macos-users-of-coming-32-bit-app-incompatibility/

Today and tomorrow, macOS users will begin seeing notifications informing them that 32-bit apps will not be supported in a future version of macOS, Apple representatives told Ars.

Starting at midnight April 12, 2018 in the user's local time zone, they will see the following message the first time they launch an app that only supports 32-bit in macOS High Sierra 10.13.4: