Subcategories are fully working! It could be a bit more obvious. On Mac, create new category in the sidebar, and drag it into another category, and it becomes a subcategory. There may be other ways to do it. No idea how it works on iOS.
If you tap a category on iOS, you see a 3 dot button appear. Tap that to make a new subcategory.
On Mac, mouse over the category, and then tap the 3 dot button.
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