Twitter has re-enabled it direct messaging capability when two users are not following each other. The recipient has to have opted in the receive direct messages for it to work.
Remember, this was tested in 2013.
A setting that allows you to receive Direct Messages from anyone, even if you don’t follow them. Twitter Re-Enables Direct Messaging Option From Non-Followers [blog.twitter.com]
Updated messaging rules so you can reply to anyone who sends you a Direct Message, regardless of whether or not that person follows you.
A new Direct Message button on profile pages on Android and iPhone. You’ll see it on the profiles of people you can send Direct Messages to.
We’re rolling out these changes starting today to all users around the world.
Twitter Testing DMs From Any Follower [webmasterworld.com]
Twitter Removes The DMs From Any Follower Feature [webmasterworld.com]