Here’s an article from a Dutch lawyer about this specific problem. Emphasis mine.
This problem was already recognized when the GDPR was introduced, and it is therefore stated in the right to forget that it does not apply if the processing (i.e. the publication) is necessary for the exercise of the fundamental right of freedom of expression (Article 17 paragraph 3 GDPR ). This means that a forum administrator can in principle prevent the removal of messages. A profile or registration of a user falls outside that fundamental right and must therefore be removed on request.