I've heard this concept described as a "honeypot field" before and it works pretty well as you've said. I'm curious how password managers/autofill avoid tripping up though; are they able to detect that the field is not visible?
I got the idea from a book. Can't remember which, but it may have been in the early oughties, or even last century (and the book called it "Honeypot Fields").
The nice thing was, the user didn't have any friction at all. They had a subject and message. No CAPTCHAS, no math problems. Nothing.
The form was on a site with a fair bit of exposure and traffic. Another similar site used Contact Form 7 (or whatever the predecessor to it was), and we would get fairly regular spams. This was a plugin that I hand-crafted.