You can get a 30 day trail at https://www.esri.com/en-us/arcgis/products/esri-cityengine/trial for Windows and Linux.
Last version update for macOS was for Mojave and High Sierra. MacOS version being deprecated: https://community.esri.com/t5/arcgis-cityengine-documents/announcement-deprecation-of-macos-version-of-arcgis-cityengine/ta-p/1038521