I believe that the iOS app started preventing these recently; I noticed that all of my Messenger notifications stopped including the text of the message, instead just stating "<name> sent a message." (or something of the like), forcing you to actually open the app.
Lets just say it was not my favorite change, and not just for the privacy reasons under discussion.
> I noticed that all of my Messenger notifications stopped including the text of the message
Isn't that a preference in the notifications setting? "Show Previews" gives the options of "Always", "When Unlocked (Default)", "Never". Perhaps it's just defaulted to "When Unlocked" after an update?
I did just check, and it is set as "When Unlocked (default)". The unlocked screen does just show a no-preview Messenger notification, the actual preview text seems to be being provided to iOS as "<name> sent a message".
I think the "seen" feature is a net positive though. Almost everyone likes it from the PoV of the sender.