Thanks to @kathywinter who pointed us to this Twitter blogpost published last Thursday:
Remember how we told you we were working on ways to let you to express more with 140 characters? Since then, we’ve introduced twoupdates, and today we’re rolling out another. Now, when you reply to someone or a group, those @usernames won’t count toward your Tweet’s 140 characters.
With this change, we’ve simplified conversations in a few ways:
Who you are replying to will appear above the Tweet text rather than within the Tweet text itself, so you have more characters to have conversations.
You can tap on “Replying to…” to easily see and control who’s part of your conversation.
When reading a conversation, you’ll actually see what people are saying, rather than seeing lots of @usernames at the start of a Tweet.
It’s now easier to follow a conversation, so you can focus on what a discussion is about, and who is having it. Also, with all 140 characters for your replies, you have more room to participate in group conversations.