GDPR is not magic, if someone saves something you put online, you're relying on their goodwill not to re-publish that later if you reconsider it as a mistake. Best outcome is achieved by thinking very carefully before you put it online, because statutory legislation cannot protect you if someone re-publishing something attributed to you is sufficiently motivated to keeping that data publicly available.
If you do something interesting enough online, good or bad, it will last a long long time. And there's nothing anyone can do about it, it is equal measures of arrogance and ignorance to believe such a thing to be possible.
If you are worrying about someone using the data that you are posting online then you should probably rethink whether you want to post it in the first place. If you are posting/sending personal data then it should always be encrypted anyway. Theymos has shown at least that hes only willing to release data that he has to because of the law but is not going to release data he thinks is irrelevant. I think in the past he has released information only on big cases such as the silkroad messages.