If you’re talking about suing Reddit for copyright/intellectual property law infringement, unfortunately, that’s unlikely to happen. Reddit can claim certain rights over user content because it’s not against the law to sign over those rights through a user agreement. It’s a bad idea, but it is likely to be considered legally binding. The “right to be forgotten” under GDPR is a specific form of control of content that can’t be signed away, but it’s not about copyright.