He's banned on HN for frequently being aggressive and using offensive language, especially the N-word (which may not mean the same thing to him as its historical meaning). Some on HN feel that banning for this reason effectively discriminates someone due to their mental illness. He sometimes posts good content and has valuable insights.
The argument against un-banning is that you can still read his posts if you have an HN account and turn showdead on. Terry is an esoteric personality, but many of us are aware of his work and enable showdead for this reason. The posts aren't entirely censored.