tarsnap natively protects against inadvertent or malicious deletion or corruption — old tarsnap backups are immutablez The low-cost competitors (restic, borg, etc) seem to have this feature as an afterthought, and they make it surprisingly difficult.

(FWIW, S3 can be somewhat straightforwardly configured so that old data is effectively immutable. Google Cloud Storage’s similarly named versioning feature appears to be far weaker.)