Hi,
I don't see an existing Idea or Enhancement request for this. That being said, currently a CAPTCHA would be of limited use in that it would be able to protect the web page from loading automatically but would not protect the feature service submission endpoint, which can be reached outside of the Survey123 application. This is still a topic of research for the Survey123 team; if a solution to that issue can be found, it would make a CAPTHA implementation a viable enhancement.