datomic has both soft delete (retraction) and hard delete (excision).

He was working in a health startup and this was something like a couple of years ago. He said that datomic was immutable so actually deleting things to HIPPA standards was not possible I think. They might of fixed it by now.

Having installed a medical records system for live-fire use by an optometry practice, I can say that it's exceedingly rare that you ever want to delete anything that is logged in a medical records environment. It's so rare that it's never been done on the four or so years I've been maintaining the system.

Datomic is an immutable database, so if you need ephemeral (deleteable) data you could just set up another non-Datomic store.

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.