From reading the GitHub privacy statement, I'm interpreting that private repositories won't be used for AI (https://docs.github.com/en/site-policy/privacy-policies/gith...). I'd be interested to hear any different though.
So I assume that if you were to supply a significant chunk of code to AI and ask for a code review, then your code could end up in the training data even though it wasn't written by AI (unless you know there's a manual opt-out option [2]).
[1] https://github.blog/changelog/2026-03-25-updates-to-our-priv... [2] https://github.com/settings/copilot/features