Twitter has referenced an "Abusive behavior" rule they implemented in April. It explicitly states: "we do not tolerate content that wishes, hopes or expresses a desire for death, serious bodily harm or fatal disease against an individual or group of people."
https://www.techspot.com/news/86970-twitter-delete-tweets-hoping-trump-death.html?utm_source=dlvr.it&utm_medium=blogger
https://www.techspot.com/news/86970-twitter-delete-tweets-hoping-trump-death.html?utm_source=dlvr.it&utm_medium=blogger