How do I get the password hint for an encrypted disk?

Have you tried entering an incorrect password? If I enter a wrong password three times, I get the hint displayed.

I hope this can help:

  1. Unmount your encrypted volume.

Disk Utility Unmount Disk

  1. Mount back.

(If the password is not stored in your Keychain, it will ask you for the pass in a dialog box and will display a button called "Show Hint")

Show Hint Button

  1. Press the Button Show Hint

Hint Revealed

If you upgrade to MacOS High Sierra 10.13.6 (17G65 at least), you can find the hint back when you use APFS file system with (Case-Sensitive, Encrypted) options

Do you have any older Macs? You could try mounting it on an older Mac OS. As is often the case with Apple, features that have disappeared still work on older OSes.