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.
Except it's possible to work around it, with a minimal amount of knowledge, or by using alternatives.
Reddit? Lemmy.
Twitter? Mastodon.
YouTube? Peertube. For youtube just use piped.video
Ads? UBlock.