Reddit has already clarified, your account is deleted, but any comments or posts made are left behind unless you remove them manually. The username is removed on each comment and post. The database doesn’t know who it belongs to if you delete your account before doing this. That being said, reddit only tracks the last 1,000 comments you have made. You have to go back and find any additional comments beyond that. Same goes for all other content such as your posts, saved posts from others, comments you upvoted, etc. The internal limit is 1,000. Additionally, the database keeps one revision. So even if you delete it, an admin can restore it if they so desire. The username will be missing of course. The delete counts as a revision state. You first edit your comment with something random, wait 24 hours, and then delete it. If an admin attempts to restore your comment, it will simply be whatever gibberish you put in.

As a result of all this, Reddit is not 100% GDPR compliant.