@ben Content on SO was always licensed under CC BY-SA licenses. Various sites legally scraped it for years. It's like Wikipedia in this regard. GDPR does not apply here. SO and those other sites can keep your contributions as long as they comply with CC.

Because of SO's terms, they might also have a special proprietary license that allows sublicensing. Contemptible, but legal.

You are sadly right about Discord, Twitter, etc. This is why community-run projects like Fediverse are so important.