This is separate from MDL-63821, which aims to make v3 available in all of the places where v2 is currently supported (eg signup form, contact site support).

ReCaptcha v3 is significantly different in its approach to v2 (using a JS call to determine a score and then requiring some decision on the action based on the score, with no direct user interaction), so to integrate it via the existing way v2 is injected into pages requires some more investigation and a deeper implementation. By first implementing this as an MFA plugin available as part of the login process, we can gain a better understanding of how it works, in isolation from the methods and limitations of the existing v2 setup.