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.
Commercial social media that is free does and will track your activity on the site
I don't think many people have an issue with this. It's all the bullshit they pull to track you off the site that is the problem, and they do a lot of that.
Edit: and the selling of that information to third parties that you would never consent to. That's also a huge issue.