I mean that RuntimeBroker.exe was considered so important that the UI process (Explorer.exe?) couldn't proceed to show the start menu while it wasn't running, and instead had to wait for it to run.

That RuntimeBroker couldn't restart until the last instance was dumped is what's true for all processes. But that doesn't explain why the UI block waiting for it and not show the start menu.

RuntimeBroker may be required to show the complete/correct start menu because it relates to app permissions for store apps and similar, but Microsoft (again) likely overestimated the importance of their app store.