Clicking the “unsubscribe” link from a notification email takes a user to a page where they can alter their notification preferences. For example:
If a user selected either the “stop watching all topics in the <category_name> category” or the “Don’t send me any mail from Discourse” options, you will see the changes on their preferences page. Either the category will have been removed from their watched category list, or their email preferences will be changed so that all settings on their email page will be set to “never”.
It’s more difficult to see the change if the user selected the “stop watching this topic” option. In that case, you could impersonate the user from their admin page, then visit the topic to see if their notification level for the topic has changed from “watching” to “tracking”. You can also get the information about their notification level for the topic from the Rails console:
Before getting too caught up in this, go to the site’s Skipped Email Logs page. You can navigate to it from the admin sidebar. Enter the user’s email into the “To Address” field that’s at the top of the page. It might show you details about why the particular email was not sent to the user.