I'm a little confused as to whether something changed in a recent WordPress version to break plugins like http://wordpress.org/support/plugin/wp-email-login that overrode these restrictions. I'm also unsure of the status of that particular plugin, so I figured I would come at this from a different vantage point:
1) Are overrides to permit email addresses as usernames still possible in recent WP versions?
2) Is there some sort of documented way to allow this, if not some ready-made plugin?
3) What is the reason for the restrictions in the first place?
4) If there was a change that broke these older overrides, what brought that on, and is this still kind of a moving target?