To my knowledge, it was broadly rolled out, I don't use anything by old.reddit.com and it's rare, but it doesn't seem limited to a specific sub-reddit. Ads, for reddit is also bought at the reddit.com level, and each sub-reddit is a monetizable segment with unique demographics. There were reps from reddit ad sales that were telling me how excited they were with what they were working on ads/tracking pipeline several years ago, and I've seen more tracking get implemented since.