There is an exception to the “right to be forgotten”, which is stated in GDPR article 17.3
Paragraphs 1 and 2 shall not apply to the extent that processing is necessary:
- for exercising the right of freedom of expression and information;
Recital 65 #5
However, the further retention of the personal data should be lawful where it is necessary, for exercising the right of freedom of expression and information
(which implies that the retention of personal data is lawful, even when the data subject has withdrawn his or her consent)
and this can be used for forum owners to retain the actual forum posts.
(source: Dutch internet laywer Arnoud Engelfriet, see article in Dutch)
Obviously, it would be good if the forum owner would be helpful and redacted any kind of obvious personal information that a user does not want to share anymore. But that’s common sense.