You can use the GCS Bucket Lock feature. It is very permanent and cannot be unset even by Google employees. That means if you set a 5 year retention policy, that GCS Bucket, and the GCP account hosting the bucket is permanently locked for 5 years. That is 5 years of GCP bills with zero way to stop it. Best practice is to set very short lock periods like 30 days, lock periods can be extended but never shortened or canceled.