Twitter adopted a limited duration shareholder rights plan, often called a “poison pill,” a day after billionaire Elon Musk offered to buy the company for $43 billion, the company announced Friday. The board voted unanimously to adopt the plan. Under the new structure, if any person or group acquires beneficial ownership of at least 15% of Twitter’s outstanding common stock without the board’s approval, other shareholders will be allowed to purchase additional shares at a discount. The plan is set to expire on April 14, 2023.