Here is an interesting read.
It says below about "the Right to Erasure":
Wait. Posts? Yes, the “Right to Erasure” does not apply to posts made by users on your forum.
When your users join your forum they accept the Terms of Service, and the ToS indicate that they will grant you a license to all the content they publish on your forum.
And GDPR article 17.3 says that the right to Erasure shall not apply to the extent that processing is necessary for exercising the right of freedom of expression and information.
But still, this is the first forum software that I know of that doesn't allow deleting PM's.
Have experience with MyBB, phpBB and vBulletin.