Twitter isn't "banned" in China in the way Westerners conventionally understand the term. Twitter IPs are blocked as a matter of policy by the GFW. Various institutions inside of China have the IP exception rules like universities and government offices. If you have an officially sanctioned reason to use Twitter like buying ads on there for Western export products, you can apply for an official VPN that gives you legal access to Twitter. Twitter has a sales office in Beijing that helps with stuff like this.
Selling unsanctioned VPN services is a crime in China but using an unsanctioned VPN is in a legal gray area but largely tolerated so long as you don't draw attention to yourself. Twitter itself has estimated that it had 10M MAU inside of China as of 2016.