One example might be audio captchas. They're needed, of course, but it means there's two avenues open to attackers now.
From an accessibility perspective, the only solution that makes sense is pervasive surveillance to determine if you're human or not.
I actually remember reading a post saying that an accessible CAPTCHA is hard. To make it accessible, you have to make it machine-readable, which defeats the point...
So captchas should only be hard enough to make complicated setups involving ML models or pipelines to Mechanical Turk not worth it. Pervasive surveillance is an overkill for this particular purpose.