Since nearly the whole Google is blocked in China, https://www.google.com/recaptcha is not an exception. However, recaptcha.net is offical and accessible from China. Just change

https://www.google.com/recaptcha/api.js?render=explicit

to

https://recaptcha.net/recaptcha/api.js?render=explicit

in front end, and

https://www.google.com/recaptcha/api/siteverify

to

https://recaptcha.net/recaptcha/api/siteverify

in back end, and it should work as expected.

We may add an option, alternative_recaptcha in Caddy plugin, --alternative-recaptcha in CLI, to switch to recaptcha.net from www.google.com as host in both recaptcha handling (backend) and serving (frontend).