It’s an interesting idea, but I’m not entirely sold on it, to be honest. Maybe there’s muscle memory on my part, for sure I’ve gotten used to the white/blue (or black/blue for dark themes) split in these screens, but to me that’s become a part of the Discourse identity as well. It feels like a big step to remove it.
Let me post some side-by-side screenshots, it helps to put things in context (they are different instances, but it’s just to give the idea of the change).
Further, in the sign up modal, the “Create Your Account” CTA is behind a scroll, you don’t see it right away.
Also, something else to think about, in all the full screen login screens the logo of the site is missing. Users usually know where they are, but the logo is an important additional validation that you’re indeed entering sensitive data (an email, a password) on the right site. In the old screens it’s there, sort of, courtesy of the modal frame.