Twitter has two sets of rules for users — loose ones for public officials and more restrictive ones for everyone else. Trump relies on Twitter's "public interest protections" to tweet threats, lies, and hate speech that would get anyone else banned. But he will lose those protections when Biden assumes the presidency, according to Twitter.
From The Verge:
President Donald Trump will lose Twitter privileges he enjoys as a world leader when President-Elect Joe Biden takes office on January 20th, 2021. Twitter confirmed that Trump's @realDonaldTrump account will be subject to the same rules as any other user — including bans on inciting violence and posting false information about voting or the coronavirus pandemic.
Twitter applies special policies to world leaders and some other officials, leaving rule-breaking content online if there's "a clear public interest value to keeping the tweet on the service." The public interest policy was formalized in 2019, codifying a rule that had been informally enforced for some time.