Hidden categories can be seen in the list of viewed categories
At the entrance to:
Both other users and visitors can expect.
Through this you can see all the categories of the forum also hidden categories.
Of course it is not possible to open and see the content of the posts, but the list of categories can be seen.
Thanks for reporting https://github.com/NodeBB/NodeBB/commit/cf6eadb91a6d6b543429be64751b6f7d6e454363