As mentioned here, the GDPR gives people the right to have their PII removed from a system.
They can request removal of all PII data from a system, which is very time consuming for admins and may very well have significant effects on the system itself, but it’s also possible and, given that this forum is run by volunteers, very reasonable, to only request anonymizing their posts and account, so that the published information cannot easily be linked back to them.
When anonymizing the content, there will still be traces of PII in the system, e.g. IP addresses in logs, backups, etc. But to most of the world, it’ll require a fair amount of work to link back the content to an individual.
I would hope that everyone participating in this forum understands the efforts it takes to handle these cases, and agrees with anonymization rather than complete removal.
Perhaps it would make sense to add some text related to this this to the forum terms and conditions. It is not possible to override the GDPR, but we can at least set the expectations right, so that people know what to reasonably expect from the fine people running the forum before sending of content.