Recently, the U.S. National Labor Relations Board has ordered Tesla CEO, Elon Musk, to delete a tweet from 2018. The tweet implied that employees would lose their stock options if they formed a union. The US NLRB deemed that this tweet was illegal and needed to be taken down. The NLRB also directed Tesla to offer one former employee reinstatement as well as to rescind 2017 rules that prohibited distributing union literature in its parking lot on non-work time and rules that barred distributing union stickers, leaflets, and pamphlets without first obtaining permission.
See "Tesla CEO Ordered to Delete Anti-Union Tweet", Reuters Staff, Reuters, March 30, 2021