TL;DR: Reddit is removing the option to opt out of ad personalization, targeting ads based on user activity. Some specific ad categories can still be limited, but there's no more opt-out option.
And may it usher in the era of antisocial media. The era where we all stop trying to impress each other, monetize each other, and just buy a thing and have it do the thing we bought it for, and not be bothered. Long may it reign.