There are, Dang has talked publicly about some of the mechanisms they've used to try and deter bad posting. Most of them are gone iirc (such as shadow banning and artificially slowing down the site) but rate limiting is still one of the mechanisms they have in place.

I guess you've never been greeted with "You're posting too fast!" on your first post of the day.

Depends on what you mean by hidden restrictions. If someone's ability to vote is disabled without notifying them, and they can still upvote or downvote but it has no effect, would you call that a hidden restriction?