Google Photos backup option for messaging apps is now turned off by default
There's a reason that Google decided to turn off the backup and sync option with these messaging apps, and it's not hard to guess: COVID-19. As per Google's statement, “due to COVID-19, people are sharing more photos and videos. To save internet resources, backup & sync has been turned off for device folders created by messaging apps like WhatsApp, Messages, and Kik.”
That being said, Google Photos users can still turn on backup and sync settings at any time within these apps. Even with the option turned off, you should be able to find photos and videos from any of the messaging apps that you're using on your device and choose which one to back up.
Keep in mind that you'll be getting a message in the Photos app when this change is added, so you'll know when to turn it on again if you still want to be able to back up your photos and videos.