Versions of macOS after 10.13.4 may warn that “‘Mathematica’ is not optimized for your Mac.” when starting Mathematica.
macOS 10.14 Mojave is the last macOS release to 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.