Where can I download ArcGIS Diagrammer?
ArcGIS Diagrammer is not supported with new version of ArcGIS Desktop. Instead, you can have look on X-ray for ArcCatalog :
Domenico Ciavarella has been maintaining an updated version here: GitHub - nicogis/Geodatabase-Diagrammer: Geodatabase Diagrammer ArcGIS for Desktop.
The link above is for Geodatabase Diagrammer rather than ArcGIS Diagrammer.
Please check this link: ArcGIS Diagrammer 10.3
Check the comments towards the end..