I downloaded the same recaptcha just now from same link suggested above. However, it does not show under checkout legacy turned off. However, I just discovered that on checkout if you select 'Registration' it appears.
It does not show for guest or login. Maybe log in it shouldn't. However, I think it should for guest where it ask for name and email. It does show for guest when Legacy is on.
In Hikashop Recaptha V2 plug in, I tried access public and guest as well, but no show under guest checkout.
So I did figure something out. If I put for Captcha after Email then it shows for guest. So because guest did not have password, it must not think it needed it.