The Google app has received a new update that lets you interact with more messaging apps with voice commands. With your permission for the Google App to access notifications, Google will now read your messages to you from third-party messaging apps and let you reply to them via the Google Assistant.
You could already do this with text messages, but the Assistant was never able to read your messages to you from WhatsApp or Facebook Messenger, except on Android Auto.
To get started, ask Assistant to “read my messages” and it might prompt you to allow access to Notifications. Once granted, it will read your messages back and ask you to reply via voice. WhatsApp, Telegram, Slack, GroupMe, Discord, and Facebook Messenger are among the list of apps that are working.
The Google Assistant visual card shows you a preview of the message, the name of the contact, and the app it will reply with. This appears to be a fairly recent addition to Google Assistant. Try it out and see if it is working.