My thinking is that these days, the unsophisticated bots will still be stopped by literally any effort, like a hidden form field that causes the form to be reje

My thinking is that these days, the unsophisticated bots will still be stopped by literally any effort, like a hidden form field that causes the form to be rejected if it's filled in. Almost nothing will stop sophisticated bots, and nothing will stop a boiler room. This doesn't really leave a place for more sophisticated CAPTCHAs.