Twitter didn't make much fuss last month after letting users opt in to get direct messages from people they don't follow . Maybe they knew it wouldn't last.
The option has disappeared from Settings, and Twitter is referring questions about the feature to a blog post about experiments. It's unclear how many people opted in or whether opening the floodgates caused a jump in spam. I opted in last month and didn't receive a single piece of spam, and I doubt others experienced a significant increase.
Twitter's direct-messaging change seemed like a step forward for the social network, which is famously slow to alter its core products. Some have speculated that Twitter is ending the experiment because it has decided take a cue from Facebook and develop a stand-alone messaging app, but take those rumors with heaping piles of salt. Twitter isn't much for fads, and is highly focused on promoting its essential function, public conversation, in the wake of its IPO. Pivoting to a chat app would be the trendy thing to do, but Twitter has bigger fish to fry right now.