The forum owner should create a simple privacy policy that details what information is gathered; why it's gathered; how long it's kept; and how to correct mistakes.
If someone does report the forum to the regulator all that'll happen is that a letter will be written asking the forum to come back into compliance, and giving advice about doing so.
GDPR makes many references to proportionality. It also mentions standard practice.
EG here: https://gdpr-info.eu/art-24-gdpr/
> Taking into account the nature, scope, context and purposes of processing as well as the risks of varying likelihood and severity for the rights and freedoms of natural persons, the controller shall implement appropriate technical and organisational measures to ensure and to be able to demonstrate that processing is performed in accordance with this Regulation. 2Those measures shall be reviewed and updated where necessary.
> Where proportionate in relation to processing activities, the measures referred to in paragraph 1 shall include the implementation of appropriate data protection policies by the controller.
> Adherence to approved codes of conduct as referred to in Article 40 or approved certification mechanisms as referred to in Article 42 may be used as an element by which to demonstrate compliance with the obligations of the controller.
