Twitter seems to have changed its mind about allowing private "direct messages" (DMs) between users who don't follow each other.
A few weeks ago, the social network quietly changed this requirement by allowing users to opt in to receive DMs from any users, even those users they didn't follow. The idea seemed to be a play for more useful personal communication via tweets and perhaps a simpler way for power users like journalists to hear news from sources.
Now Twitter has removed the option to opt in. When one Twitter user asked for the reason behind the change, Twitter shared a link to the "Experiments @ Twitter" site, which explains that Twitter sometimes tries out new systems on its users.