The GDPR increases liability on forum owners, it has nothing to do with XenForo or the responsibility you think it legally has over the content in the RM. You are choosing to host that content on your website, if that means your website mishandles personal data that liability falls on you, as a data controller, for using software that allowed that to happen. I'm sure the software takes some liabilities as well, but really, I'd imagine most the risk is on you. You're trying to push it away onto XenForo and add-on developers when that's just not the case with the GDPR - it has no effect here and really we're just going off topic with the issue.
And talking about the issue, I really don't see the problem in this thread anymore. XenForo's current limited moderation of the RM seems enough to me. I don't see major issues with resources posted in that section, and hence don't understand people's complaints here.