[foooorsyth]

The opposite, actually. Signal endlessly nags you to turn on notifications, and when you turn them on, previews and content are shown by default. You cannot opt out of the nags.

[commandersaki]

According to my setting screen the Show Previews setting is "When Unlocked (Default)".

Screenshot of notification settings page: https://files.catbox.moe/3gwjoy.png