There's the line in there about "usernames cannot begin or end with an underscore." It's stated as an absolute, that it's not allowed. But it is, sort of -- we get a lot of regreens about how they see such usernames all over, how can they not be allowed?
Can we tweak that line to say something like "Though it used to be possible to create usernames that began or ended with an underscore, this is no longer allowed due to subdomain compliance issues" or whatever the correct 'reason' phrase would be.
Something to acknowledge that yes, you will still see them all over (because they always will -- I think far more people 'kept' their underscored username than those that changed it) but it's no longer an option.