I’d like to correct some confusion and misinformation in this reply. First, Nabble was really just a wrapper around the mailing list, and everything sent to the mailing list is archived. The forum mentioned in the original post was a completely different one, and their information is lost forever. It disappeared long before GDPR existed.
Second, it’s not true that sending mails to the mailing list (in the past via Nabble/email or currently only via email) leaves no trace besides emails in your email box: all mails you send to the mailing list are archived and can be browsed (and - to some extent searched) in the mailing list archives. Use the “browse archives” links here https://supercollider.github.io/community/mailing-lists
But since the discourse forum is not legally allowed to use the existing mailing lists as backend, I too am curious how this forum will prevent just disappearing in thin air when the owner decides it’s too much work, or too expensive, or … just disappears for whatever reason.
Just my 2 cents of course