Their response on twitter [1]: "The recaptcha, when it shows up (in rare situations), is sandboxed so that it doesn't send any data to Google. We are also in the process of replacing it with hcaptcha." Not sure what possible sandboxing they could be referring to - if they load the captcha in an iframe from a different origin then it is true that Google's javascript can't access things on the Protonmail origin, but th…
also, captcha in general shifts burden onto and penalizes legitimate users, especially privacy-conscious ones, in addition to malicious ones. that is, false positive rates are too high to achieve acceptable false negative rates.
it would be better not to use a centralized captcha service, if one must be used at all.