After 126.96.36.199, the View > Diagram menu action was removed from the Data Modeler, but there is no documentation explaining why this was removed.
The Data Modeler Diagram functionality was removed in the 3.4.2 release, and will remain removed in the future. But no release note was added to explain the change, and why. Essentially, the diagram functionality was redundant, and caused browsers to crash if many associations were added to business objects. The Association Manager is the superior solution to view the relationships between business objects.