Same as IC8/CC4, the content manager has been upgraded and called the Smart Content Manager.
Basically, the user can't install content via the Downloader (installcontent.rlcontent), nor the legacy .exe installer. This can only be done through the Smart Content Manager.
See this manual on how to download the content:
https://manual.reallusion.com/content-manager/2.0/04-downloading-contents/downloading-contents.htm