No, I don't think captchas are a reason people are leaving platforms, at least not in any large number. If the content of a platform is good, the (extremely slight) roadbump for people is not going to deter them.

Keep in mind that for the average person (and even the average techy), they will not even see the captcha, because they have enough tracking them across the web that it will 'autoresolve' them as a human. I only see captcha's nowadays when I am working on web scraping automation or browsing from my VPS.

Buddy, I a close a tab the instant I see any form of popup. Add friction, lose me as a consumer of your content. Screw that noise and the psychopaths in marketing.