And I just learned that Workflowy has added kanban functionality, with kanban boards sitting inside outlines.

Even more interesting are aliases they call ‘mirror’ items - and you can mirror kanban items anywhere in the outline as outline items

https://www.youtube.com/watch?v=CiO0Ody6MfI

Unfortunately the kanban implementation seems to be limited to three columns across. Probably enough for most personal uses but it could be insufficient for some.