Removing the ability to opt-out of ad personalization based on your Reddit activity, except in select countries.
Reddit requires very little personal information, and we like it that way. Our advertisers instead rely on on-platform activity—what communities you join, leave, upvotes, downvotes, and other signals—to get an idea of what you might be interested in.
The vast majority of redditors will see no change to their ads on Reddit. For users who previously opted out of personalization based on Reddit activity, this change will not result in seeing more ads or sharing on-platform activity with advertisers. It does enable our models to better predict which ad may be most relevant to you.
edit: updated link to old.reddit.com
As some of the comments point out, they already have a precise list of interests per user.
The subreddit lists. Yet they aren’t using that data. Although I suppose the “why not” is that they maybe don’t want advertisers knowing all of the subreddits.