Hi

To be honest, I'm not sure if Facebook's Messenger is part of the iPhone backup. But, if it is, then you had a backup done while the content was on the phone, and you have not done another backup since then, they could be there. You could try restoring that backup to your phone to see if it recovers that content. Not all apps are programmed to put their content in the area that the iPhone backs up on the phone. You might also want to check Facebook's support files for Messenger. If you deleted this content in 2018, not just that the content was from 2018, it is gone.