The v1.1.0 minor release generalizes the semantics of the DigestSet field type to support any type of immutable identifier. This change is backwards compatible because cryptographic digests are strongly recommended to achieve immutability, so any implementations that only support cryptographic DigestSet still meet the modified semantics. The _type of a Statement is still https://in-toto.io/Statement/v1
More details in the updated DigestSet spec