It works when manually going to https://www.reddit.com/account/register/ (both on mobile and desktop), but it doesn’t work when following the Log In / Sign Up links from the Reddit page, which have additional URL parameters attached. Maybe this is also an A/B testing situation, as the additional parameters have “experimental” in their name.
I might be thinking of old.reddit.com. I know that the mobile version requires an email there but the desktop version asks for it, but if you click continue, it doesn't check if you've entered one.