If Notification summaries are available for your Pixel, the option will appear under the Manage section of the Notification ...

If rolled out to your phone, go to the Settings app > Notifications for “Notification summaries” at the top of the list after ...

If you just want the TL;DR, your Pixel can now give you a summary of your longer conversations. Google explains that lengthy texts across chat apps are easily recapped in your notification shade, ...