For anyone who is especially privacy-conscious:
Now private messages offer end-to-end encryption! You and the person you are messaging should exchange a shared encryption key -- preferably over another encrypted communication channel.
Once you have a shared key, you can use that key to encrypt and decrypt messages sent through the forums private messaging system. If you use a strong enough encryption key, it's a very good way to ensure the messages can never be seen by anyone who doesn't have the key. The key is never sent to the server; it performs encryption and decryption entirely client-side (in your web browser).
Even without the use of the additional end-to-end encryption feature, your private messages are secure and fully encrypted on the server. This new feature is simply an additional layer of security for added peace of mind.
Remember: Any messages encrypted with an end-to-end key will be forever unreadable if you lose said key. I cannot access them as an administrator.