Twitter no longer counts mentions against character limit
Twitter gives users who respond to an existing tweet more space. Mentions are no longer counted towards the character limit, allowing all 140 characters to be used for text.
When users respond to a tweet, the tweet no longer identifies who the message is addressed to. That state from now on in a line above the tweet. These mentions, seen as ‘@username’, are therefore no longer counted in the character limit.
Twitter announced the arrival of the change along with a number of other changes back in May 2016, but has now actually implemented it. Previously, users were given more space for tweets containing links to images or videos. Those links no longer count towards the character limit since September last year.