If you want to run a social media platform, GDPR is infringing your freemdom of speech. You can argue that it’s worth it, for the illusion of more privacy. I just don’t think it is.
If on your social network someone wants his posts to be removed, you have to comply under GDPR, or else. HN for example doesn’t allow to remove your comments after some time.
If it's a part of conversation - like most of social media posts -, I would assume it's fair use to keep it. Like if I interview you, and I publish the video, you can't retract what you said. Why social medias should be different?