As with Instagram, the change does not seem to have affected sharing outwards, but will impede users’ ability to follow friends that they may know from Twitter.
This is part of Twitter turning the screws on sharing information about the users of its network out to other services like Instagram (and by extension, Facebook) and Tumblr.
Or as Marco Arment put it:
This looks like some serious bullshit on Twitter’s part
I know Tumblr and Twitter have always had a good relationship and Tumblr was even working with Twitter to be a major Cards partner.
So this is how Twitter treats its “partners”?