Look into the Retention Lock feature.  It does require a license.

what is the backup tool being used, as depending on the product there might be true integration with the DD retention lock features (either governance or the more strict compliancy lock, which cannot be undone at all unlike the governance lock). For example Dell Networker (Avamar is to follow still for full integration) would be aware and in control of the settings, whereas 3rd party backup products might require a different approach meddling with the and setting locks. Dunno myself how automatic retention lock would exactly play into that nowadays.

https://www.dell.com/support/kbdoc/en-us/000079803/data-domain-retention-lock-frequently-asked-questions-faq?lang=en

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