Could deciphering on client similarly like mega(dot)nz does work?
Discussion
Nope, the keys would be public in posts, and that strips away any plausible deniability
Well, it would need hosting providers to scan the Nostr network and actively monitor content beyond their servers. With Mega, they do not see the content, nor can they. They have to be notified by some party to remove the content, and the notification must include the key. But that kind of defeats the same blossom ID being independent from the provider. Well, in the end, if you want to host Epstein files, IPFS would be the best choice.
I think most file storage altcoins (storj, filecoin, whatever) give plausible deniability by never storing the full file on one single node, so that storage providers never know what they are storing.