Immutable/offline backups. If you backup to local physical media (HDD/tape), physically disconnect/eject it and store it somewhere safe. If you back up to cloud storage (S3, etc), many of them have immutability options. If configured properly nobody (not even you) can delete or modify the backups (within the specified time period).
This is why Google has been using their browser monopoly to push their "Web Integrity API". If that gets adopted, they can fully control the client side and prevent all ad blocking.