add a way for form validation to require confirmation rather than changed input

Seems this was answered in a support request ticket https://www.drupal.org/project/drupal/issues/2933004 →

Now if they are different please reopen updating issue summary for D10

Thanks.

That support request doesn't sound like the same thing -- a confirmation form is a second step.

What I was proposing here was something that works similarly to a validation error, but lets you continue with the value that caused the problem.