Earlier quoted context omitted.
Datomic treats all data as immutable, so it can wind back to any version. When new data is written, the entire block is copied and rewritten rather than changing the data in-place.
That would be difficult to use given requirements like GDPR's user data deletion.
Datomic describes its capability here: https://docs.datomic.com/on-prem/reference/excision.html