Comment 1 for bug 595648

You're correct that the documentation is now out of date. However, plymouth is only the /default/ interface used by cryptsetup; you can probably get the same effect as before by using a 'keyscript=' option in /etc/crypttab. Though plymouth is still going to control the console, so you'll have to do some further tuning if you want to be able to unlock using either local console or a remote unlock command.