Good. 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.

In the Signal app itself there's an option to hide the message body or both the sender and body, that way the OS wont have anything to store in the history.