I asked Autodesk directly about this issue and received the following response. It would seem this feature is gone. Luckily, a new one is in the works using forge. It seems they retired this tool due to all of the content already existing in the content libraries that come with the software.

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Previously, the download content command would launch and bring you to a site  called "Building 3D Content" The site has been discontinued, and the command will be removed in later versions. This is why nothing happens in the 2019 version, but the command is returning unrecognized in the 2021 version. 

This feature has been discontinued, see the comments from our Development team below:
 

In an effort to provide a better experience, we are working on a new MEP Fabrication content service, utilizing modern interactions and technologies built on the Autodesk Forge platform.  To maintain focus on that goal, Autodesk is retiring the content download service, http://www.building3dcontent.com/.  This portal was also accessible from commands and menu items within CADmep, ESTmep, and CAMduct. 

Analysis of usage of the portal over the last several months of active use indicates that over 99.9% of the content downloaded was already available as part of CADmep, ESTmep, and CAMduct, and thus, downloading  from the site was redundant.  Additionally, it has been discovered that the in-product commands “Download Content to Here” and “Help > Download Content” are not functional when using the products with single user license subscription.   

We look forward to developing a more modern approach for managing content.  To stay abreast of public information related to this initiative, please refer to communications via the Revit Roadmap.