Just to be clear, the ability to automatically back up your photos to the cloud hasn't been removed; you can still activate the feature to continue uploading pictures automatically and Google has even provided the relevant instructions to perform the action.
Don't fret, though; if you're clever, you can reenable the auto-backup option for all of your device's photo folders.
Google Photos Turns Off Backups for Third-Party Services
Google Photos has stopped backing up images and videos from folders created by social media and messaging apps like WhatsApp, Messages, and TikTok (via Android Police). This change is already in effect and Google says the Photos app will give users a message describing the same as well. In an official post, Google said that the decision has been taken to save internet resources as people are sharing more photos and videos during the pandemic. Guidance on how to do that is offered in the Google Photos Help Center. Tap them, and then tap the slider to enable "Back up & sync".
More details are available in an associated help document, but the very short version is that Google Photos is changing its default setting to disable backing up content in device folders created by apps like WhatsApp, Messages, and Kik. Your Photo library should always get backed up no matter where the photos in it came from.