don't just
take, give.™

Note that the 3D Warehouse no longer supports direct downloads for the 2018 version format. You may need to download models as .collada (.dae) files or use a newer version of SketchUp to "save as" a 2018 file.

If you are looking to understand the features, system requirements, and the reality of finding a "free update" for this specific version, here is a comprehensive guide. Why SketchUp Pro 2018 (v18.1) is Still Popular

SketchUp Pro 2018 v18.1 remains a powerful tool for 3D designers who value a streamlined, local software experience on Mac OS X. However, as macOS continues to evolve, maintaining compatibility with this legacy software becomes increasingly difficult. For professional work, it is often worth exploring the latest versions to ensure you have access to the 3D Warehouse and the latest rendering capabilities.

The search for remains active among 3D designers who prefer the classic "perpetual license" feel of older versions or need compatibility with legacy hardware. SketchUp has long been the gold standard for intuitive 3D modeling, and the 2018 release was a landmark update for macOS users.