https://www.reuters.com/article/us-tesla-nrlb-musk/tesla-ceo-musks-anti-union-tweet-from-2018-must-be-deleted-u-s-labor-board-idUSKBN2BH3BV >Tesla Inc Chief Executive Officer Elon Musk's 2018 tweet threatening employees would lose their stock options if they formed a union was illegal and should be deleted, the U.S. National Labor Relations Board said on Thursday.