Also found this blog post and made the request to Reddit for GDPR data removal. However I removed comments by using the Power delete suite (https://github.com/j0be/PowerDeleteSuite).

After few hours got email (similar as in the blog post) from reddit and followed by the instructions given in email (basically send as a reddit message confirming account deletion). Few hours passed again and got confirmation email from Reddit “We’ve now flagged your account for irreversible deletion. Please allow some time for this action to complete.”

Basically not much effort needed in case you’re willing to remove the comments (etc) manually or using scripts, extensions, etc.

However not sure if it would be GDPR compliant for Reddit to leave the comments if the user data is wiped out even if the username is removed from the posts.