Do not use Google's reCAPTCHA.
It has come to my attention that ProtonMail uses Google's closed-source reCAPTCHA engine to verify new users before they are accepted into the platform.
This is a privacy risk.
It allows Google to monitor ProtonMail registrations. This also is a fundamental risk to security. To provide your users with proper safety and privacy, it would be best to remove the Google reCAPTCHA functionality from your website.
For more information and possible alternatives, please visit this article on Reddit: https://old.reddit.com/r/opensource/comments/gt2cvf/protonmailopensourceor_recaptcha/