It's now possible to hide notification previews only while your phone is locked, and for every last app on your phone. (howtogeek.com)
The «Always» option is the default and will always show full notification previews, even on the lock screen. (howtogeek.com)
It's worth noting that while notification badges are relatively consistent among launchers and Android versions, notification previews do vary by launcher. (androidcentral.com)