Um. Android has notification history also and I see no similar ability to hide notification content from the system ...
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 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.