We were a bit surprised by the sudden reaction today. We have been using reCaptcha as one tool (among many) to fight abuse for years now. For example, here's a thread from 4 years ago mentioning it [1]. It is triggered most often for signup, but it can also appear for password reset, username lookup, sending mail, payments, login, and any other api routes which can be abused.

That said, we can also understand the reaction. Back in 2014, there were no viable alternatives. Today, there is one alternative, and we started the transition to hCaptcha earlier this year, and will complete it in the coming weeks.

For security reasons, we can't say too much, but some truly massive residential IP botnets have appeared in recent years and can make millions of attempts per day. On really bad days, Captcha can appear for nearly 1% of legitimate users (some who are unwittingly part of the botnet), while blocking nearly all of the malicious attempts.

[1] https://www.reddit.com/r/ProtonMail/comments/5z70cd/when_sig...