GDPR does (fortunately) not grant the right to dictatorially sabotage a tree of content at the hand of one user, if exercising this right is in conflict with other peoples interest.

In the case of StackOverflow post are a contribution to the collective knowledge base rather than personal data. If your posts do not contain personally identifiable information, StackOverflow can easily argue that the content does not fall under GDPR’s scope for deletion based solely on personal data grounds.