Use https://github.com/NodeBB/nodebb-plugin-category-optin but beware if you have thousands of categories and millions of users this plugin will create a lot of entries in the database.
Dealing with unverified users in the ACP
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I think Latest Users means, or should mean, the same thing as Verified Users. In other words, if an account has not been verified, it's never listed under Latest Users.
Along those lines, an option that will allow the admin to enable auto-purging (auto-deleting) user accounts that have not been verified after X number of days would be nice, too.