Since Trimble no longer lists 21.0.0 as the default download on their main site (as of 2025), here is the official method:
The release of SketchUp 21.0.0 marked a significant milestone for Trimble and the SketchUp community. As users continued to explore and create with the new version, one thing was clear: SketchUp remained a leading 3D modeling software, empowering users to design, build, and communicate their ideas with ease. sketchup 21.0.0
Sometimes old faithful is all you need.
If you are a freelancer with a GTX 1060 laptop, or a firm with legacy plugins, hunting down a legitimate license for version 21.0.0 is a smart move. It will remain relevant for architectural visualization and woodworking design for at least another two to three years. Since Trimble no longer lists 21