I assume that Reddit keeps a list of IP addresses and advertising buyers can correlate them with data from other sources to associates accounts with real people.
Presumably, a Reddit leak means even more opportunity to unmask (dox) users who have responded truthfully to threads that say things like "what's the worst thing you ever did". Lots of blackmail opportunities.
It’s worse than this. Reddit, several years ago, introduced outbound click tracking.
All outbound clicks from the site are redirected via out.reddit.com which ties click activity to an individual (username / IP / device fingerprint based). This can only be blocked with aggressive old.reddit script blocking which breaks portions of the site.
The outbound click data is used for profiling and interest based advertising, but the data can be much worse than simply linking comments to identity.
They also tie IP/identity to individuals to serve relevant ads and this is the push to get people installing reddit on phones, where deviceIDs is an easy UUID for ads.
Did that finally go out to all subs, because for a while it was only on certain subs? I don't use Reddit anymore, after they suspended my account for promoting a peaceful protest of the Billionaire's Summer Camp, where the corporate media meets every year to consolidate the industry and plan the years narrative. This, after they allowed me to be harassed, threatened with sexual violence and doxxed. Good times!
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.
Presumably, a Reddit leak means even more opportunity to unmask (dox) users who have responded truthfully to threads that say things like "what's the worst thing you ever did". Lots of blackmail opportunities.