Yes, Facebook has discontinued displaying conversations messages in email notifications and that's kind of disappointing for many users.
Does Facebook provide an option to revert this?
Not as of now. We are just stuck with it and for as long as Facebook / Messenger wants us to.
This is a good privacy move because many attackers / hackers find it easier to hack email accounts and servers than a Facebook account so given that a message is sent over email it makes it easier for those who sole aim is to intercept message, which might be private or confidential, so Messenger is strictly keeping it on the app.
On the other hand it is also a good marketing strategy for FB / Messenger to prompt user to use their app more often.
I am pretty sure long ago I created a Facebook app to intercept inbox messages and send it to my email address whenever I got a new message or someone sees my message but I lost my previous account and the app and either do I have the code but looking at the Facebook Graph API it doesn't seem possible now, Facebook has strict regulations on what an app can access. So I am not sure if there exists an app that can help.
Facebook is very "concerned" about privacy, or at least wants to seem like and that might be one reason why they have taken these steps.