[1]: https://www.linkedin.com/pulse/nightmare-letter-subject-acce...
It gave 2 year to get conformity to something which has been effective since 1995 through a European directive. The forum owner decided to ignore this a do nothing, now he's confronted with the consequences of his choices.
But again his reaction is akin to a knee jerk reaction as he is probably in capacity to answer the request while outsourcing to a third party does remove his responsibility and obligation to answer GDPR requests just now it's gonna get a bit more complicated as he has a third party into the loop which is probably not complying either.
You might want to get in touch with fluxBB dev to get a GDPR button extracting the relevant data from the database to avoid the pain of building the db query by hand.