A new type of relay has just appeared. It deletes events right after they have been 'queried' by a user. Useful for very specific applications where no traces are desired, like shh.com from  nostr:npub1zxev3yxc0vr2wwldstsmjzknug6mk8uwhsw7k6ka78e3c4mqvc7qxue9y7

https://github.com/Dolu89/ghost-relay