An article on The Verge reports that Twitter has started sending emails to owners of inactive or unused accounts with a warning saying that they should sign in by December 11, or the account will be deleted and its username be made available again. This will affect accounts which haven't been signed in for more than six months.
Twitter made this move in compliance with their policy on inactive accounts, which is summarised as:
|"We encourage people to actively log in and use Twitter when they register an account. To keep your account active, be sure to log in and Tweet at least every 6 months. Accounts may be permanently removed due to prolonged inactivity."|