You're correct that Datomic considers immutability a feature. That said, Datomic on-prem supports excision which completely removes data for exceptional cases. I suspect Datomic Cloud will sometime in the future.
With EU's GDPR (General Data Protection Regulation) looming on the horizon I think Cognitect would do themselves a disservice should they not include excision in their Datomic Cloud platform sooner rather than later.
