Or to be more precise, why was this moved to the interstitials step in the registration?
I'm referring to this commit.
I don't really see why this was needed. If the forum requires a valid email address, why not just show that at registration, and not as an after thought? Even if you tick "Require new users to specify an email address" at /admin/settings/user#user-registration, you only get prompted in the optional email input during the interstitials.