It is not an entire plugin but it's the only code you need in a plugin to achieve what you want. A plugin has some boilerpate code. You can see a full starter plugin at https://github.com/NodeBB/nodebb-plugin-quickstart. The plugin docs also go over the details https://docs.nodebb.org/development/quickstart/

Currently watching categories is opt-out. On forums having lots of new topics, this results in a huge number of items in example.com/unread. I think the unread page would be far more useful if watching categories was opt-in. That way /unread would contain only the items a user is interested in from the start and they won't have to go ignore a bunch of categories in order for /unread to become useful.

@baris yes we should add that to core, it is amazing then, that way we can find people who r following similar interests