Kanban is actually the default mode for next gen projects. Sprints and backlog are features you can toggle on or off in project settings.
If you're asking if you can add an extra kanban board on top of a next gen project with sprints enabled, that is not possible indeed in next gen. All features are linked to your project there and they are either on or off.
You can build a filter with your project issues and build a classic kanban board on top of that as a workaround.
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