GDPR protects any data which can be tied to an identifiable person. So, if reddit posts are attached to my username, and my username can ultimately be tied back to me (see: my username is my name), then reddit cannot disallow deletion of these posts. There may be a workaround by no longer identifying them with a specific user, but this would have to be done on the back end, as simply changing who the displayed user is would still leave the data intact.