You can undelete your account for a while. They probably can’t allow that option if you go the GDPR route. However, they still must keep data as required by law. Those periods differ by country.

I guess that the routine anonymization before training is the same as after deletion. GDPR isn’t copyright. They have to delete that data which makes the content personal to you, but not the content.

The main effect is probably that the data becomes exclusive to Meta and unavailable to others.