Inactive Twitter accounts will be removed in December, freeing up these usernames for reuse.
Twitter is sending out emails to owners of inactive accounts with a warning: sign in by December 11th, or your account will be history and its username will be up for grabs again. Any account that hasn’t signed in for more than six months will receive the email alert.
Twitter hasn’t yet said exactly when recouped usernames will be made available to existing users. The account removal process “will happen over many months — not just on a single day,”