Displaying the workspace name would be a step in the right direction. I imagine that would easier to implement due to the way Vivaldi treats windows. It seems Vivaldi treats windows as quasi-transient containers, so if you named a window "Shopping", then changed the workspace on that window to "Home", you'd likely want the label on the Window to be "Home". But would the tabs you had open before you switched to a workspace still be associated with the "Shopping" label? Then they would be sort of an ad-hoc workspace. I don't know what Vivaldi calls these tab groups.
So I think this can be implemented in two steps:
When on a workspace, use workspace name for window
Allow naming of the group of tabs open in a window that are not associated with a workspace.
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But the lack of this feature is a major issue. I find I can really only have one or two Vivaldi windows or my productivity plummets, so I end up having to use other browsers for actual browsing and Vivaldi for things that don't change much.