alt Hacker NewsGood. The moment they add it, all kinds of apps will start to abuse it, for "sekhurity" (read: engagement) reasons. See e.g. all the apps that now disallow taking screenshots, for no legitimate reason.
Personally I'd be in favor of a hard app store policy, that if an app notifies you about something, all the importantdetails (like full message text) must be included - specifically to allow the user to view the important information without having to open the app itself.
I'm referring to what sounds like a feature of the app, not the OS... The app... already chooses what to send through the OS notification API so I really don't have any idea what scenario you're worried about.
I generally sympathize, I also don't like when apps block screenshots (or even more stupidly, they can block Android's amazing "select text from anywhere" feature...). But I don't think there are similar concerns for Signal allowing me to hide notification content from the OS.