Earlier quoted context omitted.
I seem to remember being pretty upset when I found out Terry Davis of Temple OS was being shadowbanned. Maybe it was just temporary for a short period or a single comment, but I didn't like it. I found this (related?) poll https://news.ycombinator.com/item?id=7818823
Terry was permanently shadow-banned, the subject came up quite often. He suffered from schizophrenia and would make racist and paranoid comments about 80% of the time, but his remaining posts were often pretty technical / insightful. As the victim of an illness it seemed unkind that he was banned, but at the same time a lot of his output was obviously very offensive.
Incidentally, if a banned account is making only good posts, we're happy to unban it. I often look at the recent commenting history of banned accounts in the hope of finding such cases, and users sometimes email us about them (as mirmir mentioned elsewhere in this thread). That's super helpful!
One strange phenomenon is that there are banned accounts that post good comments, but revert to posting bad comments that break the site guidelines as soon as we unban them. Then we ban them again and their comments get good again...go figure. Any large-enough population sample includes a long tail of behaviors.