I think GDPR won't change anything for most forum admins. Most even don't collect personal data from users (public posts are not "personal data") beside their email addresses. And those are deleted once the account is deleted.
Regarding newsletters, I don't think the new law requires to change anything. You should allow your users to unsubscribe if they don't like the newsletter. The interpretation of some lawyers that you have to allow unsubscribing without a login is very questionable. Professional admins will of course respect unsubscribe requests via email answer to the newsletter and make it generally very easy to unsubscribe.
I also don't think Xenforo has a dedicated "Newsletter" feature.
Regarding newsletters, I don't think the new law requires to change anything. You should allow your users to unsubscribe if they don't like the newsletter. The interpretation of some lawyers that you have to allow unsubscribing without a login is very questionable. Professional admins will of course respect unsubscribe requests via email answer to the newsletter and make it generally very easy to unsubscribe.
I also don't think Xenforo has a dedicated "Newsletter" feature.