It's probably in the privacy policy / terms of service that you don't read when signing up (no one does...). It's actually nice of Reddit to announce these sorts of changes and not just say "we updated our privacy policy"
Reddit has a large community of technology savvy users what looks better "We started doing X that some of you may not like" vs "We updated our privacy policy"..few days later "They started doing X, burn them!".
I wish more companies would be smart.
It's not even in their announcements.
This message is only in /r/changelog, and going backwards they first introduced this 4 months ago, had to roll back and have been rolling it out again with other options.
This is not a nice way of introducing what is, to me, a fairly large privacy change.