Do you know if there is a geodatabase diagrammer for arcgis pro?
To my knowledge, no there is no ArcGIS Pro equivalent of Geodatabase Diagrammer. I did find an old version (ArcGIS Desktop 10.5) located here that may be of some assistance.
We're moving to ArcPro later this year, but the loss of Diagrammer will be a big problem. Any updates on this?
Not to my knowledge.
GitHub - AlexArcPy/registrant: Python package used for generating HTML reports about the contents of... provides the closest functionality, in fact, a little better IMHO.
While not replacing the same tool, the Generate Schema Report tool might help. It sounds like the future plans include a data model diagram option.
