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This emphasis on a single unified list represents a shift in our thinking over time. Previously we’d assumed that we needed separate taxonomies for different groups of users and different types of content (mainstream/specialist/policy). We now think a single taxonomy which includes all content and meets the needs of all users will make it easier for users to get to the content they need, and for publishers to classify their content.
More reading
https://www.nngroup.com/articles/utility-navigation/
https://www.nngroup.com/articles/flat-vs-deep-hierarchy/
https://www.uxbooth.com/articles/the-rules-for-modern-navigation/
https://bilberrry.com/notes/why-we-use-task-based-navigation/
https://www.nngroup.com/articles/audience-based-navigation/?lm=utility-navigation&pt=article
https://www.nngroup.com/articles/ia-questions-navigation-menus/?lm=utility-navigation&pt=article