I decided to replicate my post from support forum here and suggest a new helpful feature.
Currently “Netlify-provided reCAPTCHA” can not be linked to the submit button and it doesn’t make sense because the user can just ignore it and submit the form. And Netlify will ingore such submissions. Result: the important letters are lost.
I propose to allow to add the custom attribute to the button. And when the site is published, include the necessary code to disable the button until captcha is passed.
Something like this:
<form name="contact" method="POST" data-netlify="true" data-netlify-recaptcha="true"> ... <div data-netlify-recaptcha="true"></div> <button data-netlify-recaptcha="true" type="submit">Send</button>
Then generate the code on your side for the button, like you do for the captcha div.
It will be very handy feature.