After the violent events at the US Capitol on January 6, Twitter announced it was permanently suspending @realDonaldTrump for violating Twitter Rules.
Specifically, Twitter assessed two Tweets he sent on January 8 under its Glorification of Violence policy and found that “they were highly likely to encourage and inspire people to replicate the criminal acts that took place at the U.S. Capitol on January 6, 2021.”
More information on Twitter’s stance against glorifying or encouraging violence can be found here.