Hmm. It make sense that the option is not offered if all users already have the category in their sidebar.
You could apply the step of removing the category to everyone. After that adding should trigger the “apply to existing users” modal. But this might be confusing for users currently using the forum.
You can also check the current config using data explorer. This query allows you to select a category and a group and returns for every group member if they have the category in their sidebar
-- [params]
-- group_id :group
-- category_id :category
SELECT
u.id AS user_id,
CASE WHEN sl.id IS NULL THEN 'No' ELSE 'Yes' END AS category_added_to_sidebar
FROM users u
JOIN group_users gu ON gu.user_id = u.id
LEFT JOIN sidebar_section_links sl
ON sl.user_id = u.id
AND sl.linkable_type = 'Category'
AND sl.linkable_id = :category
WHERE gu.group_id = :group
ORDER BY category_added_to_sidebar ASC
The result looks like this
