Can the GoJS UndoManager be saved so that it can be used again for undo and redo after loading the model next time?
In general, no, because it has references to arbitrary non-model Objects that are not serializable. That includes every GraphObject that is modified, and those GraphObjects have references to their Diagram, and the Diagram has references to the DOM.
It’s something that we should investigate. An UndoManager that only has Model ChangedEvents might be serializable.