This is the "Contributors" feature, which has been tested internally and with a few partners, but has never been widely enabled. It's in the public API, but still disabled for all but internal accounts and those few partners. https://dev.twitter.com/docs/api/1/get/users/contributors

I don't have any current insight into the status of Contributors, and won't speculate, but the feature has been stagnant for almost 3 years now (just like Lists).