If you just need to quickly edit an old .123dx file you have lying around, go ahead and install the legacy version. But for new projects? Save yourself the headache. Embrace the modern tools.
You can find an Autodesk 123D Design download for Mac on archive sites, but only for macOS Mojave or older. For macOS Catalina and newer, you must use a virtual machine or embrace modern alternatives like Fusion 360 or Shapr3D. Autodesk 123d Design Download Mac