Clients can do whatever they want to filter content. If you don't like that than use a different client. Damus doesn't do this because it can easily be gamed. In your case it looks reasonable because people do not seem to like your schizo posting. It is not censorship because there is not a central authority making this decision, its just users on the network flagging your posts because they don't like them. They are hidden in amethyst but no other clients. So no there is no "shadowbanning" on nostr because its not possible to shadowban on nostr.