But what Stephen said:
Freedom of expression is more broad. Reiterating what I said before:
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(which implies that the retention of personal data is lawful, even when the data subject has withdrawn his or her consent)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)