Hi,
As fas as I know there's no (never been) a seperate viewer download/installation.
Next to the Revit full version there should also be a "Revit Viewer xxxx" icon shortcut.
It's just a command switch in the shortcut starting the full version into the viewer mode.
like: "C:\Program Files\Autodesk\Revit 2019\Revit.exe" /viewer /language ENU
AutoCAD has a seperate viewer download.. (DWG Trueview I believe)
HI @MikeRhead
Were you able to get this resolved?
As for an answer no, Autodesk didn't remove Viewer Mode. It's still part of Revit
Mike