By anonymizing you are removing all the key fields to identify the person, just not what they wrote.
My sites have Creative Commons BY SA licences. Anything someone writes on a site goes under that licence. If someone joins the forum they should be clear what that licence means which in my view is handing over the right for everyone (including the site) to use that data as per the licence conditions. Withdrawal of that data is not negotiable as the licence is irrevocable.
See Can You Revoke a Creative Commons License? - Plagiarism Today
So it’s important to get the licence right in the first place and probably where you should take your guidance as to whether the person has the right to ‘take back their posts’.
Perhaps CC & GDPR are not 100% compatible. I did a search and have not yet found an authority that discusses that fully.