David Duke, who led the KKK from 1974 to 1978, has been permanently banned from Twitter for breaking the site’s rules forbidding hate speech. The exact posts haven't been revealed, but Twitter prohibits promoting violence or threatening attacks against people based on religious affiliation, race, and ethnic origin. Last month, Duke's YouTube page was suspended for violating its hate speech policies.
When Twitter was recently questioned about why Duke, who joined the site in 2009, hasn't been suspended yet, they stated that he is not currently a KKK member and "has distanced himself from the organization publicly." Duke was previously suspended by Twitter very briefly in March of 2017, but the reason behind the suspension was never made public.
Source: The Hill