I can only imagine that 6 or 7 years ago (when I imagine 18 was being specced) it all seemed "good enough".
Maybe they just forgot about it until LUKS itself receive a major update. Tends to happen in these large projects, you just don't go around turning stones on stuff that appear to work.
Though at the loss of plausible deniability!
If someone finds a wallet on your system with private keys in it, I think they can reasonably assume the coins are yours. The fact that they have access to the public keys would allow them to trace the movement of the coins too. To improve protection, the new wallet format allows you to encrypt all data in the wallet (not just private keys) with a second passphrase.
This still means the attacker can see your public keys in your local blockchain database. Encrypting that data is a lot more complicated than the wallet, I haven't got around it yet. You'd have to use a supernode to avoid that data leak for now.