Versions of macOS after 10.13.4 may warn that “‘Mathematica’ is not optimized for your Mac.” when starting Mathematica.
Apple announced that macOS 10.13 High Sierra will be the last macOS release to fully support 32-bit applications. Mathematica, Wolfram Desktop and other Wolfram Engine–based products still use a 32-bit front end on Mac at this time. Wolfram Research is working on the release of a 64-bit Cocoa-based front end. Until that version is finished, Wolfram Research will fully support High Sierra by developing, testing and shipping versions with our current front end technology.
We will update this article when we are able to provide more information about the forthcoming release of the 64-bit Mac front end.