Validation may not be possible (Because it requires an external service for example, or validation rules may not even be decided. For example. I suspect that many MVP demos have a login/signup, which might validate the email. I also suspect that e.g. minimum password rules are enforced, it's usually done much later).

But yes I agree it's a poor example if it's possible to do right away it must be done right away. But I think you get my point. RTL-input may be the better example: it's a nice-to-have for some people, probably not part of an MVP but still a likely part of what some stakeholder calls critical functionality if you support RTL input locales.

My point is that while you may be able to categorize some things as "critical" or "important" (i.e. things one can argue should not reach the mainline even during development) there will always be a gray areas where you can't complete the development, but should still have a functioning app in some sense.