I see that you do not understand GDPR completely. You are extracting fragments detached from the whole. At any place, GDPR does not treat IP, for example, differently from other data described in the definition. At any place, GDPR does not treat private and public personal data. In both cases, these are personal data and are protected by law.
GDPR requires the right to be forgotten, this also applies to the archives of databases. It is currently one of the biggest technical problems - archives may require older versions of systems. Read what troubles big companies have in this area.
GDPR does not require anything from the person whose data it processes, it requires everything from you. You must show relevant information, collect consent, provide rights, etc. The user does not have to either remember their posts or browse them and look for what was in them. GDPR transfers this to the administrator. Admin is responsible for providing information to the user if he is in charge and asks what data he has. Admin must delete or anonymize this data. Not a user.
The only thing I'm hoping for is that really run out of resources and time to check everyone.
Good night
