You can no longer receive direct messages from random followers on Twitter. The experimental feature, which gave users the option to receive messages from any follower, regardless if you follow them back or not, quietly went live last month, though it has already been pulled without much explanation.
In response to the feature's absence, Twitter pointed to a blog post from September, which said that it's constantly experimenting with features; the social network didn't say why it chose to pull it, and whether or not it'll eventually return. Perhaps people were getting too much spam.
Apparently, Twitter is trying to revamp its Direct Message service. One rumor even suggested the social network is considering developing its own messaging app; this experiment may have been part of that process.
Twitter obviously thrives on community discussions and being able to spark conversations across the platform, but sometimes people prefer to have private conversations, away from the public eye. Letting users DM any person they follow certainly would have made Twitter feel more open and accessible—and maybe less embarrassing—but it appears that feature is gone for now. If anything changes, we'll be sure to let you know.