Where did the categories go?

Labels and categories have been transformed into Labels, which you can find here: Menu — Data — Labels. As a result, to apply a label to a task, you now need to set a label for the task. Similarly, for categories, you need to set a label to establish a category. One tool instead of two. To display categories as before in the navigator, you need to enable the setting ‘Settings > Additional > Display labels in navigator.