Twitter seems to have pulled support for its Google Glass app, removing the Glassware listing from MyGlass entirely. Glass users (including a few folks here at Android Central) have noticed that while the Twitter app continues to function if you already have it installed on Glass, it no longer shows up for installation nor does the listing in the MyGlass app have any information on it.
Although the app continues to function for now, it's only a quick flip of a switch on Twitter's end to make it stop working altogether. There's no post from Twitter indicating why the app has been pulled, but considering the low number of Glass users out there relative to other wearable platforms — to say nothing of phone and tablet users — it's likely Twitter just doesn't want to devote any more resources to the project.
With Android Wear notifications coming to Glass there's less need for dedicated Glassware these days, but until a presumed full transition of Glass to Android Wear comes you'll be left with no option for outgoing tweets on the headset.