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signed by youSigned by a private key which has no connection to my legal identity, and which can be generated in a split second. Signatures give strong pseudonymity.
> Relays don’t have to store any message on disk
This is a good idea, as long as messages are relatively few, which may be hundreds of thousands if they are short. There's no promise of availability, so an infrequent reboot is free to erase them.
> Then you are going to convince a judge that the thing on my disk belongs to me
I'm afraid it can be the other way around, if the material is sensitive enough. You'd have to explain to the judge how it ended up on your computer.
> every server in the world has this vulnerability
No, only a server that allows users to upload UGC without requiring enough background information to let the law enforcement find out their real identities if need be. Should be an adequate explanation to the judge in a civilized country where governmental censorship is not a thing.