I am obviously not a lawyer but I don't see how Reddit is in the wrong here. On GDPR.EU that "The EU’s GDPR only applies to personal data, which is any piece of information that relates to an identifiable person. It’s crucial for any business with EU consumers to understand this concept for GDPR compliance." I don't see how your comment history would be considered "personal data".
It even says in Reddit's TOS that "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".
You've agreed that your posts are no longer your "personal data" at that point...
When you delete a Reddit account, it will mark your username as "[deleted]" so they are at least attempting to anonymize the posts. Reddit has no obligation to remove anonymized posts unless it contains identifiable personal data. (https://www.jdsupra.com/legalnews/eu-general-court-examines-data-1532025/) "If data about individuals is processed so that the individuals cannot be identified, the data can be used free from the restrictions imposed by the GDPR (e.g. enabling a pharmaceutical company to use patient data for R&D)." If the data recipients (readers) can't link it back to a identifiable person (a specific person), it's not personal data. Of course, they're not going to just blanket delete every post a user ever made because that's not in their favor. If there is a specific post with personally identifiable data Reddit is clearly assuming the onus is on the user to request deletions of specific posts that contain identifiable personal data (which GPDR.EU says they are absolutely allowed to do). Unless they are challenged in court, they ain't gonna do jack shit. Not saying you can't try or that what Reddit is doing is right, but good luck!