Hi Trevor,
Yes, we were aware that Crainiate's ERM Diagram, the technology used by ArcGIS Diagrammer, went open source a few years ago. While open diagram appears almost identical to ERM Diagram it is lacking some or all layout classes. But more importantly, open diagram is no longer maintained or supported.
Just prior to last year's user conference, Jack Dangermond made the following comment with respect to Esri's support for CASE tools:
ArcGIS will continue to support CASE tools through the ArcGIS 10.3 release. However, CASE tools may not be shipped with future releases of ArcGIS that follow 10.3. For users interested in geodatabase design with UML, we recommend using Sparx Systems’ Enterprise Architect as it offers many advantages over the CASE tools functionality. UC Q&A | 2014 Esri User Conference |
As such, I would encourage you to evaluate Enterprise Architect for your design efforts.