Federico Viticci, writing on MacStories:
Similarly to Twitter’s iPhone app, Tweetbot 4.6 doesn’t count usernames against the 140-character limit. To present this change in functionality, Tapbots has opted for a Twitter-like design where usernames aren’t displayed in the compose box upon starting a reply. Instead, a “Replying to…” banner at the top of the screen highlights the tweet’s original author and other participants in a conversation. […]
Unlike Twitter’s official apps, usernames are still displayed in the body of a tweet in both the Timeline and Mentions views, providing a familiar format that doesn’t force you to tap on the “Replying to…” banner from every section of the app.
This is why I continue to use third-party clients — I can actually trust them to do the right thing with their user interface decisions. The same can’t be said about Twitter itself.