GDPR is great, but it would probably not apply to posts and comments as it pertains to personal data (name, address). User provided content is not generally personal data as per GDPR (though might contain some). Positing personal data is even explicitly disallowed in most subs.
One could argue they need to delete username, although it is in principle arbitrary, but they could be allowed to keep post/comment content. And this already happens as part of account deletion.
On top of this, users agreed to transferring rights when accepting TOS:
“When Your Content is created with or submitted to the Services, you grant us a worldwide, royalty-free, perpetual, irrevocable, non-exclusive, transferable, and sublicensable license to use, copy, modify, adapt, prepare derivative works of, distribute, store, perform, and display Your Content and any name, username, voice, or likeness provided in connection with Your Content in all media formats and channels now known or later developed anywhere in the world.”
Source: https://www.redditinc.com/policies/user-agreement/
Don’t confuse this with me liking what Reddit is doing, it’s a douchebag move, so we have all the reasons to distrust Reddit and be angry. I’m just trying to add nuance to the issue.
Obligatory IANAL
Elon Musk: “Hold my beer”