Google has rebuilt its Google Sites platform and replaced its 'Classic' site features with a new version.
You can learn more about the differences between Classic and New Google Sites.
Classic Google Sites shutdown
Starting January 30, 2023, Google will begin to automatically convert all remaining classic Sites to “new” Sites. Your classic Sites will be unpublished and held as drafts for your review. Once you complete the conversion, your classic Site will be deleted.
To check whether you are an owner of a classic Site and start the upgrade process, use Google's Classic Sites Manager.