@google's reCAPTCHA served via the domain google.com is known to be inaccessible in🇨🇳 due to censorship. As an alternative option, @googledevs suggests replacing it with recaptcha.net. This domain is now also blocked by the Great Firewall via DNS poisoning.

Given (1) many websites use ReCaptcha.net and (2) the way GFW poisons DNS resolutions with public IPs of foreign organizations, including @meta @Twitter etc. These orgs (though unrelated) may see many HTTP(S) requests for ReCaptcha.net.