Starting tonight, macOS High Sierra 10.13.4 will warn users when they launch a 32-bit app and prompt them to this support document. This was announced by Apple a while back, as Jason Snell points out at Six Colors:
Mac developers were told at Apple’s developer conference last year that macOS High Sierra would be the “last version of macOS to run 32-bit apps without compromise,” and many of them have been busy updating their apps to be fully 64-bit compatible. Just a couple of months ago BBEdit, one of my favorite Mac apps, updated to 64 bit.
For now, these apps will still run; this warning doesn’t impact the usage of these programs in any way. However, starting with the next major version of macOS, 32-bit apps will run “with compromises,” although Apple hasn’t made it clear what those compromises will be.