I have no idea if any NFT is encoded this way though.
Sure, but that just moves the problem around; instead of secret data you have a secret private key to manage. The cyphertext can be stored anywhere—on the blockchain, in IPFS—but the private key to decrypt it remains separate from the NFT: If you put it in the NFT then everyone would have it.
> Selling such piece of data would mean decrypting it with old owner's private key and re-encrypting it with new owner's public key.
Any protocol such as this would require out-of-band communication and carry counterparty risk. What happens if someone sells the NFT but refuses to hand over the data, or claims not to have it? The smart contract can't deal with this as that would require making the key public. I'm not seeing any way that an NFT improves the process over simply paying someone to give you a copy of the secret.