When user decides to delete the data they will simply remove decrypting privileges.

By telling apps that they should no longer show a certain encrypted data. You'd have to trust apps to keep the keys safe and to honor your request.

Perhaps a separate decryption service could be built. That entity and the author would be the only entities in possession of the key needed to decrypt a piece of content, each of which would be encrypted with a unique key.

Perhaps the blockchain should not hold the text content at all but only a hash of it against which to test every version that potentially claims to be the original. The content could be self-hosted by the author him/herself or on some sort of peer-to-peer networks if additional resistance to censorship is needed.

There is no reason why an app like this using Hive could not be developed.