They could nuke all posts, right? If I remember correctly r/piracy did this once in fear of the sub being delete by reddit itself because of legal issues.
Reddit probably has backups if that happens, but still..
Posts never really get deleted. Deleting them as a mod unlists them from the sub, deleting them as a user unlists them from the sub and profile. Users can edit their posts to remove the text body but the post title and link stay accessible permanently to people with the url and therefore to reddit. The only thing that gets deleted (if reddit doesn't lie about it) are images attached to the post if deleted by the user themselves.
If you send a GDPR deletion request and they don't completely, provably purge all your data falling under it, and you complain to your respective officer, en masse, that's not going to look good to your bottom line. Especially pre-IPO.
So they actually might have a dedicated process for that. If they are sane. If they are not, here is your chance to make it expensive.
They can't delete it as far as I am aware. They could delete all posts and perma-private it I think, but that would be extremely tedious and the admins could revert everything a lot easier.
When you're working within their frameworks, effective protests are limited. It's why I just left.
Given the lows to which Reddit has stooped lately, I wouldn't be surprised to see Reddit respond to that by filing a lawsuit against the mod seeking nine figures in damages.